320 Dr. Gr. J. Stoney on the Limits of Vision: 



to be bordered with colour. Finally, the spherical aberration"* 

 of the eyes becomes conspicuous when we view a considerable 

 star or planet with one eye. Instead of being seen as a point, 

 it is seen as a small irregular patch with short tails from it, 

 and of somewhat different shape according as it is viewed with 

 the right or with the left eye. Now this is due to spherical aber- 

 ration co-operating with another defect which it is difficult to 

 disentangle from spherical aberration, and which is caused by 

 the light having to pass through the other layers of the retina 

 before reaching the rods and cones. These layers, however, 

 do little harm in the fovea lutea, as here they are either 

 absent or thin, so that the irregular image seen when we look 

 directly at a planet is chiefly due to pure spherical aberration. 

 Now these defects, viz. the chromatic and spherical aberra- 

 tions, including under the latter that further defect which 

 arises while the light is crossing the retina, are dealt with in 

 nature in the same way in which a photographer deals with 

 them in his photographic camera, viz. by limiting the aperture, 

 which diminishes the effect of these imperfections. We have 

 already found that the aperture of the pupil is contracted 

 as much as is compatible with the other conditions to be 

 fulfilled. Now it is evident that a certain amount of the 

 defects with which we are at present dealing, especially when 

 rendered less operative by the limited aperture of the pupil, 

 may be allowed to remain in the eye without rendering it 

 incapable of distinguishing objects separated by 1' of angle, 

 the limit already fixed by the rods and cones ; and there can 

 evidently be no tendency in evolution to effect any further 

 improvement of the eye as an optical instrument. Accord- 

 ingly, in persons with the best vision, the eye seems to have 

 been just improved up to this point, leaving its outstanding 

 defects still very conspicuous when searched for; and it is 

 shortcoming in respect to these defects which is chiefly what 

 makes one man's eyesight less perfect than another's. 



We shall next deal with another preliminary remark, 

 which it is well to make, as it will dispel the oft-repeated 

 error that there ought to be some connexion between our 

 vision and the position of the image formed on the retina. 

 It is pertinent to point this out when engaged in inquiring 

 into the vision of insects, for, as we shall see presently, the 



* If a sphere be drawn round a point of the image formed by light of 

 one wave-length, to represent the crest of one of the luminous waves 

 advancing towards that point, the whole of the crest should reach that 

 sphere at the same instant of time. There are, however, usually little 

 deviations of some parts of the crest of the wave from this sphere, which 

 defect is called spherical aberration. 



