On Light in some of its Relations to Disease. 163 



convex glasses are demanded, attempt to read for half an 

 hour with magnifying spectacles of sixteen or eighteen 

 inches' focal length, or let him study the landscape for a 

 considerable time with a pair of glasses adapted to a near 

 sighted person, and he will soon, by his aching eyes, his 

 nausea and vertigo, acknowledge that here is a cause of 

 nervous disturbance of no slight or insignificant preten- 

 sions; and when we remember that the immediate cause 

 of convulsions and other alarming nervous disturbances is 

 sometimes apparently trivial — such as a hastily-devoured 

 meal or the taking of even a small amount of food which 

 is illy digested — we cannot fail to perceive that this cause 

 of irritation, prolonged through all the experience of the 

 individual, must be abundantly potent to produce either 

 of the functional disturbances of which we have spoken. 



A form of error of refraction not yet referred to is a 

 combination of other forms, and is called astigmatism, 

 which means without a point. In this anomalous condition 

 the refracting surfaces are different in different meridians 

 of the eye, so that no exact focus can be formed, and there 

 is an irrepressible conflict going on to correct the error, 

 which can never be corrected without external help. This 

 is a most vexatious cause of irritation and prolific source 

 of nervous disturbances. 



We are now, I think, prepared to bring our two classes 

 of facts together and see if they will fit. 



We have seen, in the early part of this discussion, that 

 the tendency to nervous affections often runs in families, 

 and that the nervous disturbances manifest themselves in 

 different forms in different members of such families, what 

 is sometimes called a " neuropathic predisposition," exists. 



IS'ow, it is also true that the faults of refraction are the 

 unfortunate heritages of many families. Indeed, these re- 

 fractive conditions are as much characteristic features, de- 

 pending as they do on the form of the eye, as the form of 



