LIBRARY 
NEW YORK 
BOTANICAL 
. GARDEN 
mri MICROSCOPE. 
Vou. VI. ANN ARBOR, JANUARY, 1886. No. 1. 
ORIGINAL COMMUNICATIONS. 
ASTIGMATISM AND ITS RELATION TO THE USE OF 
OPTICAL INSTRUMENTS. 
BY ERNST GUNDLACH, ROCHESTER, N. Y. 
ae is a defect of the human eye which disturbs 
the true or normal vision in such a way that, for instance, 
a point appears elongated in one direction, or rather it appears 
ina line. It is caused by a certain irregularity in curvature of 
one or both surfaces of the crystalline lens of the eye, that is, 
the surfaces are stronger curved in one direction than in an- 
other, thus giving the crystalline lens different focal lengths in 
different directions, so that, for instance, a line may appear 
sharp and distinct in one direction but become quite biurred if 
the direction is changed. Astigmatism can be corrected by 
glasses one or both surfaces of which are of a convex or concave 
cylindrical curvature, and if, beside this defect, the eye is either 
short or far sighted, a certain combination of such cylindrical 
curvatures and their axial directions, concave or convex as the 
case may require, will at the same time correct both defects and 
make objects appear as they do to the normal eye. Indeed such 
glasses are made as spectacle glasses with complete success, but 
however, the determination of the correct formula for the 
proper construction of such glasses requires a rather complicated 
and systematic examination of the defective eye, which can 
only be accomplished by a specialist thoroughly familiar with 
this branch of science. Astigmatism is very common, and even 
the healthy eye is not free from it. The curvature of the crys- 
talline lens is a little shorter in vertical than in horizontal direc- 
tion, thus making a line of acertain length appear shorter when 
