ON THE CHOICE OF A MICROSCOPE. 165 
ground as one of the most useful of glasses; for the same 
reason that the parabolic reflector ranks as one of the 
greatest inventions of the modern microscope—the 1 in. 
as being the highest power that can be used on opaque 
objects with the best results; the paraboloid as enabling 
us to obtain even under a 1th in. objective the effect of a 
brilliantly reflected light. And I am glad to see that 
Dr. Carpenter, in his work, ‘The Microscope,’ though he 
does not lay so much stress on the advantage of medium 
powers for opaque objects as is here insisted on, nevertheless 
intimates his satisfaction at the marked attention lately paid 
by our first opticians to the construction of such glasses 
as are capable of showing, in the most perfect manner, that 
fascinating class of objects. 
And with respect to transparent objects (not to speak of the 
advantage of the achromatic microscope over the simple, accru- 
ing from the luxury of a large and well-defined field, causing 
infinitely less strain upon the eyes) , the circumstance of obtain- 
ing with the medium glasses of the achromatic instrument this 
high magnifying power combined with considerable length 
of focus is so great a convenience in the examination of 
organised structures, and of living actions, whether vegetable 
or animal, when covered with a thick medium of glass or 
water, that this alone constitutes a benefit almost as striking 
as that which is derived from such glasses when employed 
upon opaque objects. 
But it is time now to leave this part of our subject, and say a 
few words to those whose tastes lead them or means oblige 
them to confine themselves to the less expensive class of micro- 
the utmost amount of light that can be transmitted through an object-glass, 
by the increase of its angular aperture is of so much consequence for 
exhibiting the details of objects in their ¢ruthfulness, that it is difficult to 
understand how so many scientific people should adhere thus pertinaciously 
to small angular apertures. Puta }-inch objective of large, and a jth of 
small angular aperture, but both equal in other respects, on some trans- 
parent object, say one of the common rotifera: the former glass will show 
with wonderful precision the forms of its digestive system, which the 
latter, though a higher magnifying power, will only show as comparatively 
confused with one another. Again, put the same glasses on the green 
scaies (as seen by reflected light) of the wing of the papilio Paris butterfly, 
the large aperture }-inch will exhibit their lines and striz, as clearly and 
sharply as a ith glass would by transmitted light, with a reality in fact that 
the most careful management of the Bull’s-eye condenser, or the mirror and 
Lieberkuhn would in vain enable the small aperture th to equal. It is for 
this reason, then, that I prefer well corrected object-glasses of large angular 
aperture, and feel that the trouble of adjusting them and managing the 
illumination as skilfully as may be, is well bestowed, if only the result is 
ensured at last of seeing objects /uithfully represented—as they are in short 
—and not as they are xot. 
