64 THE MIcROSCcOPE. 
to acknowledge my error in giving the causes of this defect of 
the eye. The valuable and interesting ideas thus brought out 
will render greatly increased benefits to the public, and while 
I am not only willing but glad to acknowledge Dr. Stockwell’s 
superiority in his exposition of the causes of Astigmatism, I can 
not deprive myself of the opportunity of answering his objec- 
tions to my proposed plan of using an asymmetic eye-piece— 
_ the less so that these objections appear to be well founded and, 
therefore, have undoubtedly the approbation of many readers. 
The Doctor, after giving his various objections to my ideas, 
proposes to correct the difficulty by attaching to the front of 
the ordinary eye-piece, by means of some suitable contrivance, 
a separate lens of asymmetric form, which may easily be re- 
moved. He afterwards comes to the conclusion that a better 
plan would be to have suitable cylindrical lenses mounted in 
spectacle frames to be worn only when using the instruments. 
To this plan I object. 
Spectacles should never be used with any optical instru- 
ment, however great or of whatever nature the optical defect 
or defects of the eye. Spectacles that may correct these 
troubles satisfactorily, under ordinary circumstances, are always 
injurious to the proper performance of optical instruments, and, 
therefore, the wearer of spectacles should always remove them 
before using the microscope or telescope. That spectacles are 
injurious to the performance of these instruments is attributed 
mainly to the following reasons: In the first place they prevent 
the eye reaching its proper place, or proximity, to the eye-piece, 
which is the plane of the crossing of the light pencils emerging 
from the eye lens. Secondly, the generally very excentric and 
oblique position of the spectacle glass to the optical axis of the 
eye and, consequently, also 6f the instrument, greatly injures 
the proper performance of the latter. The third objection is 
that spectacle glasses add two light refracting and reflecting 
surfaces to those already existing. 
It is almost impossible for the observer wearing spectacles 
to even roughly place the optical axis of the spectacle lens, if 
worn in the ordinary manner, in line with that of the instru- 
ment. This could of course be accomplished by mounting the 
glass upon the eye-piece according to the first suggestion of Dr. 
