CORRESPONDENCE. 
247 
immersed or dry, but neither will this affect it ; it is only a case of a 
thicker or thinner front. In all these cases the aperture will he given, 
with approximate accuracy, by the ordinary method. 
Next, suppose the object immersed. It may be covered or un- 
covered, the glass may be immersed or dry, but in every case alike the 
common method will fail, and must be set aside. There is a li mi t, 
easily determined, beyond which the aperture cannot pass ; and this 
limit will depend on the medium in which the object is placed. Take 
the angle whose sine is the reciprocal of the refractive index ; twice 
this angle is the limit of the aperture. If we wish also to determine 
experimentally the actual aperture in a given case, or how much it 
falls short of the limiting value, we must use the method described 
by Mr. Wenham, which inexperienced workers will do better not to 
attempt. Otherwise, however, it may be ascertained also by calcula- 
tion, by a method which it is not necessary to specify here. 
Briefly, therefore, the limitation or non-limitation of the aperture 
is governed by the immersion or non-immersion, not of the object-glass, 
but of the object. 
Your obedient servant, 
S. L. Brakey. 
Using the ith Binocularly. 
To the Editor of the ‘ Monthly Microscopical Journal .’ 
35 , Upper Park Street, Clifton, Bristol, Oct. 19 , 1872 . 
Sir, — I was greatly surprised to hear at a meeting of our Micro- 
scopical Society that Mr. Wenham was engaged in perfecting an 
arrangement of his Binocular Microscope, by means of which the ith 
object-glass could be employed binocularly, since I have been in the 
habit of using my Boss’s ith with Mr. Wenham’s instrument (the work 
of Mr. Husbands, of this city) for some time past, with excellent effect. 
My “modus operandi” consists in withdrawing the prism until the 
dark outline of its edge crosses the centre of the field exactly, and 
placing the instrument in such a position that the light falls upon the 
mirror from the left ; by this method, and especially hy the aid of the 
condensing lens and petroline light, I can examine with very satisfactory 
results objects suitable for the -§fh. That the object is seen with both 
eyes may be easily proved by placing a circular diatom, an arachnoi- 
discus for example, in the centre of the field, so that the dark line 
before referred to crosses its centre, and by closing one eye, and look- 
ing down the tube opposed to the other, and vice versa, it will be at 
once apparent that opposite halves of the diatom are seen, and by now 
employing both eyes simultaneously the diatom presents itself in its 
entirety, and its markings can be made out perfectly. 
Apologizing for trespassing upon your valuable space, 
Believe me, Sir, yours truly, 
Samuel Smith, 
Surgeon. 
