ON THE APPARENT FIGURE OP STARS. JQl 



Thus, there is not only a want of agreement between anato- It differs great- 

 mists respecting the figure of the crystalline j but they who have y* 

 observed it with the greatest care, as Petit, find very great 

 differences in it ; differences that must necessarily affect vision, 

 and produce in great measure those variations, which have been 

 noticed in the sight of different persons by physiologists and 

 natural philosphers. 



Whatever care has been taken to ascertain the figure of the Its anteriorand 



crystalline, observers hitherto appear to have attempted only posterior sur. 



■' '^'- ^ ■' faces supposed 



to determine the proportions that exist between the versed sines to unite in a 



of the curvature of the segments, AB, AC, fig. 14, and the circular line, 

 length of their chord, DE. No one that I know has endea- 

 voured to ascertain, whether the plane of the posterior and 

 anterior segments were circular ; and whether there existed 

 any difference between its diameter from right to left DE, and 

 its height GF . This difference appeared to them not suffi- 

 ciently perceptible to be measured. 



However, as there are some crystallines, the horizontal and This not al- 



vertical diameters of which exhibit a pretty considerable ^^^^ ^^^ "^® 



'^ ■' according to 



difference, these could not escape an accurate observer. Thus Petit. 



Petit, in a paper read to the Royal Academy of Sciences in 



1730, says : " the circumference of the crystalline is commonly 



round ; yet I have found some in the human subject, that were 



not so, and the diameter of which was a quarter of a line 



longer one way than the other." 



In thi? paper Petit describes a great number of observations He examined 

 made on the crystallines of various animals ; those of man being >5 . '" various 

 introduced only as forming one of the links of the great chain. 



As Petit is the only person, who has measured crystallines The eiamina- 

 with sufficient care to perceive, that those of man are not **°" repeated, 

 round j and to observe, that one of the diameters exceeded the 

 other by a quarter of a line ; I thought it might not be amiss 

 to repeat these experiments, in order to satisfy myself whether 

 this particular observation of Petit was sufficiently general, to 

 contribute to the production of the irradiation j and at the 

 same time ascertain the directions in which the longer and 

 shorter diameter are placed. 



I immediately procured two sheep's eyes, which I opened sheep's eyes, 

 cautiously. On taking out the crystallines, and laying them 

 flat, I perceived, that the curve uniting the two segments was 

 longer in one direction than in the other. 



