THE Microscope. 3 215 
CORRESPONDENCE AND QUERIES. 
Liperty, Pa., September 30, 1886. 
Editor, Microscope. 
There is one great objection to high power objectives—from 
one-sixth inch to one-fiftieth inch 7. e. the extremely short working 
distance between objectives and cover glass. I am no expert in 
optics, but I do firmly believe that the form, the construction, the 
arrangement and combination of lenses for objectives can be modi- 
fied so as to give a working distance between the objective lense 
and cover glass of one-fourth inch for a one sixth objective and one- 
thirty-second inch for one-fiftieth inch objective. I may be looked 
on with apparently good reason as a crank for entering the field so 
boldly with such an idea. Things have been accomplished (suc- 
cessfully) apparently as difficult as this enthusiastic idea of mine. 
For example high angles of aperture of objectives. It appears that 
ninety degrees was at one time considered the highest limit of an- 
gle of aperture for objectives, and the man who predicted it pos- 
sible to construct an objective with an angle of one hundred and 
eighty degrees aperture certainly would have been called a crank, 
but it has been successfully accomplished “ just the same.” How 
much higher the angle can be applied to objectives I do not know. 
It may be the limit. Is it not possible or even probable that the 
working distance may be increased to the limit mentioned? It 
would permit the use of thicker cover glasses which might be from 
one-fiftieth to one-thirty-second inch thick—a great desideratum for 
dry mounts on which thin cover glasses are so easily broken,— 
nearly an inevitable fact. For balsam mounts thin cover glasses 
answer tolerably well, having a solid support over the entire sur- 
face. How cover glasses one-thirty second inch thick would act 
on balsam, I don’t know, or whether the unequal shrinking of 
balsam would crack them. I think they would spring that much 
' before cracking. 
Thick covers could be cleaned without the common occur- 
rence of breaking them while wiping them clean. A mount being 
one-thirty-second inch higher, owing to a thick cover glass, would 
be no serious objection so long as there is sufficient room in the 
world for slides. The frequent breakage of thin slides and thin 
covers is an annoyance which far overbalances the convenience 
claimed in the saving of space in storage or weight in mailing them. 
Another great advantage gained in objectives of such working dis- 
tance mentioned, it would to a great extent lessen the danger of 
scratching, injuring or soiling the object lense of high power ob- 
jectives. I hope opticians will make an effort in this direction. 
J. D. BECK, 
