176 Transactions of tlie Society. 



It is pleasant in these days of free thought to have the mind, 

 which has got rather into the "rest and be thankful" mood, suddenly- 

 awakened by the jarring sound of such assertions as those made 

 with regard to the binocular Microscope by Professor E. Hitchcock. 

 " The stereoscopic effects, while not of great practical importance, 

 as already stated, certainly render many objects more attractive to 

 look at. For this reason a Microscope for the entertainment and 

 instruction of friends should certainly be a binocular."* 



The Professor states that there is no advantage in a binocular 

 Microscope in studying the form of objects, but the value of this 

 opinion is shaken when he asserts, in the same sentence, that the 

 appearance of relief the binocular gives is not necessary to enable 

 us to form a correct idea of the true state of objects in which the 

 appearance of relief is most striking. 



He qualifies his opinion, however, by writing : — " It is true 

 that the binocular does reveal more of the form of an object at the 

 first glance than the monocular, but it is a matter of experience 

 that those who only use one eye in microscopical work, never make 

 the mistake of supposing that an object is merely flat because it 

 seems to be so. A few turns of the focussing screw soon give a 

 correct idea." The Professor considers that the value of the 

 binocular is restricted to the comforts of vision. I do not think 

 that these views will receive acceptation in the old world ; on the 

 contrary, it is to be hoped that the late admirable adaptation of an 

 objective of high power by a distinguished Fellow of this Society 

 to the binocular will pave the way to still more advantageous 

 developments of the binocular system. 



Constantly using objectives of low power in examining objects 

 of natural history, naturalists find that the binocular enables them 

 to decide at once whether a series of markings are elevations or 

 depressions or alternate elevations and depressions. With the 

 monocular they must shift the illuminating beam as well as alter 

 the focus, so that they can see the truth, revealed at once, by the 

 other instrument. 



In the address which I had the honour of delivering to you last 

 year I remarked upon the comparative values of object-glasses with 

 high and low numerical apertures, and I took pains to defend the 

 employment of lenses with wide apertures in examining minute 

 objects, and also to state that both kinds of objectives are necessary 

 for investigating into the structure of minute objects. I suggested 

 what has commended itself to every advanced microscopist for years 

 past, that an observer should provide himself with both classes of 

 objectives, and that he should use those with a moderate aperture 

 for common and preparatory work, and those with a high numerical 

 aperture for subsequent and careful examination. 



* Amer. Mod. Micr. Journ., ili. (1882) p. 417. 



