Picture By Laurinemily - Wikimedia

In the previous article, I have listed some of the examples to prove that animals and human do have different eye structure which provides them different ability to perform under different situations. If you haven't read that article yet, you can click the link below:

The Science Behind Photoreception (Part 1 of 3); An Introduction

Even though eyes are different when we compare them from creature to creature, it can be divided into two general types:

• Single chambered eyes
• Compound eyes

The first type is also known as a camera eye. It is a hollow structure which detects light that passes through a single lens while the latter, is a convex structure that identifies light which passes through multiple lenses. Compound eyes appear faceted when we look at it at a close distance due to the various lenses.

Single-chambered eyes

Pigment cup eyes

Tapeworm which has a pigment cup eyes - Picture By Mogana Das Murtey and Patchamuthu Ramasamy - Wikimedia

Most of the invertebrates have a set of eyes which shaped like a cup that contains a few numbers of photoreceptors (up to hundreds of photoreceptors). This set of eyes is called the pigment cup eyes. It does not have any optical system except an aperture (opening) which allows light to enter through the eyes to be detected by the photoreceptors. This aperture is microscopic, around 100 μm or less which allow only a small proportion of light to enter and react with the photoreceptors. As a consequence, the resolution is insufficient. This type of eyes is only useful to detect the incoming sources of light. When a creature with this set of eyes tries to find food sources or interacting with the environment, the only clear thing that they can see is the direction of light coming or going. Pigment cup eyes are useless for prey or predator.

Pinhole eyes

Mollusc (Nautillus) - Picture By Manuae - Wikimedia

This type of eyes has a far better resolution than a pigment cup eyes. It can be found on some species of mollusc and few members of cephalopod. They possessed millions of photoreceptor which make their eyes better at interacting with light. Their eyes also were equipped with a few muscles which allow movement and adjustment of its pupil size that will control the amount of light entering the eyes. Despite all of the improvement compared to the pigment cup eyes, they have no lens, so the image formed is still quite blurry. To make the image formed clearer, they can squeeze down their pupil to a small size to focus the source into a single location but the image produced would be dimmer than a normal blurry image. There are only two options for them to choose from, either see anything brightly but blurred or see something a little bit clear at the expense of some brightness.

Lens eyes

Goldfish - Picture By Benson Kua - Wikimedia

I think you can guess by now if a pinhole eyes have a horrible resolution due to the lack of a structure called lens, this "lens eyes" must have an improved resolution. You're right. A dense material was added in the chamber which allows a high refractive index lens to form. The function is to converge incoming light sources so that all of the photoreceptors can receive light at a reduced angle resulting in a much clear image formed on the retina. If you've realised or have observed some of the fishes eyes before, their lens appear spherical. This is to provide a brighter image by shortening the focal length of specific lens diameter. Lens eyes have two mechanisms of producing an image:

• It can form an image by moving the lens away or towards the retina
• It can control the shape of the lens by using eye muscle.

The mechanism of a lens refracting lights which caused the formation of a high-quality image on the retina is not well understood. Some of the lenses ever produced which are made from homogeneous materials could suffer from spherical aberration which in turn provides a low-quality image. The optical quality of the lens can be determined by the ratio of the focal length of the lens to its radius of curvature which is known as the Matthiessen ratio. It was speculated by James Maxwell in the 19th century that for a quality image to be produced, the lens must have the highest degree of refractory property at the centre area of the lens. This concept was observed in a few species of marine mammal, fishes and cephalopod. Even though the lens that can be found in this creature has the same refractive index with the homogeneous lens, the radius of curvature is different for both of them. According to the Matthiessen ratio, the relationship between optical image quality and the radius of curvature is inversely proportional; a high radius of curvature would yield an inferior optical image quality. In the homogeneous lens, the radius was two times greater than the radius of the lens that can be found in the animal which utilises the lens-eye system. This is one of the reasons why homogenous lens failed to produce a similar image quality that can be achieved by a lens eye creature.

Astronotus ocellatus - Picture By Andre Karwath a.k.a Aka - Wikimedia

Fish and some species of mollusc exhibit the same lens-eye system and both have round eyes which can maximise the amount of light entering for a clearer vision. Despite all the similarities, they are different in term of:

• The structure of the retina
• Photoreceptors

The retina which can be found in a fish is arranged in an inverse manner relative to the neuron which emerged at the front side of the retina. The optic nerve can be located at the back of the eye which is a set of nerve fibres assemble through an optic disc. The eyes of the mollusc, on the other hand, is everse which meant instead of it is being gathered by a few structure component before leaving the eyes, the nerve fibre leaves directly from photoreceptors which can be found in the retina.

Photoreceptors which can be found in a mollusc is made out of a fingerlike projection called microvilli which depolarised (become slightly negative) when exposed to light. The photoreceptors that can be found in a fish is made up of cilia which will become hyperpolarised when exposed to light. Both of them, however, have an almost similar ratio of cones to rod cells.

Corneal eyes

Human's eyes - Picture By Garretttaggs55 - Wikimedia

This is the type of eye structure that can be found in human and certain vertebrates. Unlike any sea dwellers which depends solely on the lens for light refraction purposes, the cornea which is curved in shape can be a metaphorical lens which is responsible for two-thirds of the eye's optical resolution. The rest of it is being supplied by the actual lens. This is crucial because the air and water have a different refractive index. The focal length is determined by the following equation:

f = nr/(n-1), where n is the refractive index of the fluid of the eye, and r is the radius of curvature of the cornea.

As any spherical refractive surface are prone to spherical aberration, human's cornea is ellipsoidal instead of spherical. This guarantees the highest curvature at the midpoint of the eyes which in turn produces the best image through central vision; peripheral vision is quite weak. Fovea, an area which has the highest photoreceptors population which situated close to the central axis of the eye, has been responsible for an acute vision, which aid in the efficacy of central vision optical power.

Spider's eyes - Picture By Opo Terser - Wikimedia

I think the most interesting application of central vision by using corneal eyes can be observed in a spider. Spider has eight eyes, 2 of which is termed as the primary eyes are pointing in the forward direction which is used to identify the member if its species for social and reproductive purposes. the rest of them which are called the secondary eyes are used for hunting, maximising detection of movement made by any prey. It's hard to miss any prey passes through with the other six eyes looking for a potential food source. The secondary eyes are considered not efficient when they are building webs as it is unfocused. It will be used to navigate for help and identifying the position of the sun. The primary eyes are at its peak of function when a spider make a jump towards its prey as it can resolve the jumping arc and at the same time locate its target. This is 4 to 5 times better than the vision of any insects so the attack might be unexpected and have a high success rate.

Concave mirror eyes

Scallop's eyes - Picture By Syug stug - Wikimedia

Although the light is refracted by a lens system in this kind of eyes, the image formed is not produced by the refraction process but rather a reflection caused by a concave mirror located at the back of the eyes. The light which is reflected by the concave mirror would be directed to the photoreceptors which would form an image. Even though scallops have 50-100 single-chambered eyes, their lens has a weak refractive power which will not allow a clear image to be developed, so the role of a concave mirror is essential in producing a good image resolution. The concave mirror has a multilayer structure which is composed of cytoplasm and guanine with a thickness of 0.1 μm respective to the spectrum of light. This mechanism of image formation can gather a considerable light's optical power, but as the light hits the retina twice, once before it was reflected from the concave mirror and the other one after, the image formed is unfocused and has a low-contrast in nature.

References and further reading materials

• Michael Land (1999, July 26). Encyclopaedia Britannica. Photoreception. Retrieved March 7, 2018, from https://www.britannica.com/science/photoreception
• Anirudh Agarwal (2018, January 30). Know Your Body. Optic Nerve. Retrieved March 7, 2018, from https://www.knowyourbody.net/optic-nerve.html
• What-when-how. Eyes and Vision (Insects). Retrieved March 7, 2018, from http://what-when-how.com/insects/eyes-and-vision-insects
• Ted M. Montgomery. The Optic Nerve. Retrieved March 7, 2018, from http://www.tedmontgomery.com/the_eye/optcnrve.html