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of the Georgian planet. 
when two will collect the rays to their proper focus. The 
hole through which they pass in coming to the eye should be 
much larger than the diameter of the optic pencils, and con- 
siderably nearer the glass than their focus ; for the eye ought 
on no account to come into contact with the eye piece ; and 
a little practice will soon enable the observer to keep his eye 
in the required situation. It is hardly necessary to add, that 
no hand should touch the e} T e piece. 
With regard to the eye glasses, when merely the object 
of saving light is considered, I can say from experience, that 
concaves have greatly the advantage of convexes ; and that 
they give also a much more distinct image than convex 
This fact I established by repeated experiments about the 
year 1776, with a set of concave eye glasses I had prepared 
for the purpose, and which are still in my possession. The 
glasses, both double and piano-concaves, were alternately 
tried with convex lenses of an equal focus, and the result, for 
brightness and distinctness, was decidedly in favour of the 
concaves. 
For the cause of the superior brightness and sharpness of 
the image which is given by these glasses, we must probably 
look to the circumstance of their not permitting the reflected 
rays to come to a focus. 
Perhaps a certain mechanical effect, considerably injurious 
to clearness and distinctness, takes place at the focal crossing 
of the rays, which is admitted in convex lenses.* 
* About the same time that the experiments on concave eye glasses were made, I 
tried also to investigate the cause of the inferiority of the convex ones ; and it occurred 
to me, that an experiment might be made to ascertain whether the rays of light in 
mdcccxv. Q q 
