﻿138 Prof. J. LeConte on some Phenomena 



tain peculiar conditions under which the first two laws may be 

 violated, but none in which the last is violated. For many 

 years I regarded these experiments as confirming the ordinary 

 doctrine. I had observed in my first experiments on the carpet 

 that each successive plane became more and more indistinct. 

 I accounted for this by supposing that both the optic axes and 

 the lenses were adjusted for vision on the plane of the image, 

 while the light diverged from the floor five feet distant. It 

 seemed to me a crucial experiment, proving the necessity of focal 

 adjustment and the inseparable association of it with axial ad- 

 justment. On recommencing these experiments a few weeks 

 ago, however, I was struck with the fact that the figures of the 

 images were far more distinct than the real figures were when a 

 small object was viewed in the position of the images. To test 

 this point fairly, I placed two bone buttons in similar posi- 

 tions and on similar spots on the pattern, and then brought their 

 images into coincidence. At first the united image was indistinct, 

 but gradually it became perfectly defined, every thread-hole as 

 clear and distinct as it is possible to conceive. I succeeded, 

 though with greater difficulty, in getting a perfectly distinct 

 image of the buttons on all the planes. It was evident there- 

 fore that the indistinctness of the figures of the image on the 

 higher planes was not the result of the want of focal adjustment, 

 but of imperfection in the pattern. The subsequent experi- 

 ments with the ruled diagram proved this beyond the possibi- 

 lity of doubt. The images in this case were obtained with 

 much more ease, and the lines were defined with the most per- 

 fect sharpness, even when the image was brought nearly to the 

 root of the nose. 



In all cases, however, the image when first obtained was a 

 little indistinct, and then gradually became clear. With un- 

 practised eyes this interval of indistinctness is considerable, but 

 becomes shorter and shorter with practice, until it almost dis- 

 appears. When the image once becomes clear it remains so ; 

 but there is then a sense, while looking at the image, of gazing 

 beyond it ; or rather perhaps there is a difference between the 

 image and the real object which we cannot account for, but 

 which is not a difference of distinctness. There is evidently 

 an unnatural condition of the eyes, which produces strain and 

 fatigue. 



There is but one possible explanation of these phenomena, 

 viz. that the optic axes and the lenses are adjusted to entirely dif- 

 ferent distances. The three adjustments of the eye, viz. the 

 axial adjustment, the focal adjustment, and the contraction of 

 the pupil, have been so associated through successive genera- 

 tions, and the association so confirmed and strengthened in each 



