﻿136 



Prof. J. LeConte on some Phenomena 



So also a and d will be seen at d', and b and c at c', and the 

 dotted line will represent the position of the image-plane. In 

 order to make this image, we must gaze through and beyond the 

 pattern until we observe the double images come together and 

 coincide, and then fix the eyes steadily. The enlarged image 

 gradually becomes distinct. 



This experiment is much more difficult than the preceding. 

 The pattern should not be too small, otherwise the difficulty is 

 very great. In former years I had often performed the experi- 

 ment with perfect success ; but the wall-papering I had used for 

 this purpose had been destroyed, and I found difficulty in again 

 obtaining a suitable pattern. I therefore constructed a pattern 

 by ruling black lines on a large sheet of paper so as to make 

 perfectly equal squares 1^ inch wide. With this simple diagram 

 my success in all the preceding experiments was really marvel- 

 lous. The coloured patterns before used form far more beautiful 

 images ; but for scientific purposes the ruled diagram is far pre- 

 ferable. With this diagram standing upright before me at the 

 distance of sixteen inches, I got with great ease seven successive 

 images on this side of the object, and one beyond. All the 

 images on this side were defined with great ease and perfect dis- 

 tinctness, although the nearest both by measurement and by 

 calculation was but three inches from my eyes, i. e. far within 

 the limits of my distinct vision. With great effort I could ob- 

 tain others still nearer. The nearest I actually retained and 

 measured was but 1^ inch from the root of the nose ; but I after- 

 wards found that there was no limit except the root of the nose 

 itself. Within three inches, however, the images were no 

 longer perfect, not from any want of distinctness of the lines, 

 but because the horizontal lines of the two images were no 

 longer parallel, but crossed one another, as shown in the figure 

 (fig. 4), and therefore could not be made to coalesce perfectly. 



The explanation of this will be given in its proper place. 

 The still nearer images, as, for instance, those within 1^ inch, 

 could not be retained ; the strain on the interior recti muscles 

 of the eye was too great. 



The image beyond the object is much more difficult to obtain 

 with clearness, especially if the object be near the eyes. At the 



