170 Mr. Young's Observations on Vision . 
eye is Kepler's. He supposes the ciliary processes to contract 
the diameter of the eye, and lengthen its axis, by a muscular 
power. But the ciliary processes neither appear to contain 
any muscular fibres, nor have they any attachment by which 
they can be capable of performing this action. 
Descartes imagined the same contraction and elongation 
to be effected by a muscularity of the crystalline, of which he 
supposed the ciliary processes to be the tendons. He did not 
attempt to demonstrate this muscularity, nor did he enough 
consider the connection with the ciliary processes. He says, 
that the lens in the mean time becomes more convex, but at- 
tributes very little to this circumstance. 
De la Hire maintains that the eye undergoes no change, 
except the contraction and dilatation of the pupil. He does 
not attempt to confirm this opinion by mathematical demon- 
stration ; he solely rests it on an experiment which has been 
shewn by Dr. Smith to be fallacious. Haller too has 
adopted this opinion, however inconsistent it seems with the 
known principles of optics, and with the slightest regard to 
hourly experience. 
Dr. Pemberton supposes the crystalline to contain muscu- 
lar fibres, by which one of its surfaces is flattened while the 
other is made more convex. But, besides that he has demon- 
strated no such fibres. Dr. Jurin has proved that a change 
like this is inadequate to the effect. 
Dr. Porterfield conceives that the ciliary processes draw 
forward the crystalline, and make the cornea more convex. 
The ciliary processes are, from their structure, attachment, 
and direction, utterly incapable of this action ; and, by Dr, 
