The phenomenon of ‘second sight’ is quite interesting as it appears to the patients to be a ‘magical’ restoration of vision after the vision of the patient becomes poor. After several years of using glasses to be able to have clear vision for near work, the patient starts seeing well again without glasses. Most of the times they attribute it to some kind of supernatural power or their prayers but this phenomenon occurs because of some changes in the natural lens of the eyes which can be scientifically explained how the vision improves after years of dependence on glasses.
In a typical example, the following happens:
1. Around the age of 40 years, presbyopia (difficulty in seeing up close objects clearly) sets in.
2. The patient starts using glasses for all near work. He/she can’t see near objects clearly without glasses.
3. As the age advances, presbyopia becomes worse gradually and the patient keeps on changing the glasses to see well. In other words, the power of the lenses of the glasses keeps on increasing.
4. With increasing age, the patient notices that he/she is able to do near-work (e.g. reading) well even without glasses. Gradually a time comes when the patient does not need glasses at all and can see up close objects and fine print well.
Here is the explanation of everything above:
1. Presbyopia is caused by loss of elasticity of the natural lens leading to inability of the lens to focus the light on the retina for clear image. This is the result of aging as the lens becomes harder in consistency. A hard lens cannot change its shape to become more convex and therefore cannot bend light rays enough to form clear image on the retina. For more details on presbyopia, read this post.
2. To correct presbyopia the patient has to use glasses which are plus lenses. The glasses compensate for the poor focusing power of the natural lens of the eye.
3. With advancing age, the lens of the eye becomes harder progressively causing more loss of focusing ability. Therefore the power of the glasses required to correct presbyopia also increases simultaneously.
4. As the lens becomes harder especially in the central part (called as nuclear sclerosis), its refractive index increases gradually. This higher refractive index of the lens itself causes more bending of light to focus the light rays on the retina without the need for the lens to change its shape (that is, without the accommodation of the lens). This is the stage when the patient does not need glasses to see near objects clearly because the higher refractive index of the lens (nuclear sclerosis) compensates for the loss of accommodation of the lens.
It is also important to remember that if the process of nuclear sclerosis continues further, the central part of the lens becomes so dense that it becomes opaque. That is when it causes poor vision and we call it a cataract. Therefore ‘second sight’ is a temporary phase in the process of the lens becoming hard and opaque leading ultimately to blindness.[First image by Anthony/ CC BY 2.0]