30 THE Microscopes. 
dentally, voices a common error and says, “ It (astigmatism) is 
caused by a certain irregularity in curvature of one or both sur- 
faces of the crystalline lens of the eye.” 
For the purpose of illustrating the relation astigmatism 
bears to the use of optical instruments, or to avoid entering into 
technicalities in definition, the foregoing statement may be, and 
probably is, sufficiently satisfactory ; but writers and periodicals 
whose every utterance should be an advance step in matters 
appertaining to education and facts in science, cannot afford to 
rest their reputations on incorrect)statements under any circum- 
stances. 
Astigmatism was first mentioned by Thomas Young* who 
considered it due to some inequality in the structure of the 
lens. Later Wharton Jones claimed that its seat was in the 
cornea. By recent investigators it is now held that the aberra- 
tion due to a difference in the focal distance of the two princi- 
pal meridians—usually the vertical and horizontal—most com- 
monly depends upon a defect or asymmetry of the cornea. This 
is termed regular astigmatism and is readily remedied by eylin- 
drical lenses, the other optical defects, such as myopia or hyper- 
metropia, having been first corrected by the use of spherical 
lenses. The aberration which is due to a difference in the re- 
fraction in one and the same meridian, and sometimes in addi-’ 
tion thereto in different meridians, is caused by irregularity in 
the crystalline lens, except when caused by opacities or scars 
the result of corneal ulcers, and is termed zrregular astigmatism 
and cannot be corrected. 
The highest degrees of astigmatism are usually found after 
removal of the crystalline lens by cataract operations and are 
then unquestionably due to asymmetry of the cornea, an un- 
avoidable result of the cutting operation. This is almost invar- 
iably regular astigmatism, requiring for correction a convex 
cylindrical lens, frequently of eight to fourteen inches focal 
length, axis horizontal, in addition to the strong convex spher- 
ical lens necessary to replace in a way the removed crystalline 
lens. 
Many of the disputes and disagreements between micros- 
*Donder’s Work, p. 539. 
