50 



THE AMERICAN MONTHLY 



[March, 



'* author has fully satisfied himself by 

 "his own study under an oil imraer- 

 " sion "2^ of Messrs. Powell and Lea- 

 " land of a Podura scale illuminated 

 "by the 'immersion paraboloid,'" 

 etc., etc. 



It is well known that the wider the 

 aperture of the objective, the less the 

 working-distance maybe, though this 

 does not depend upon the aperture 

 alone. A reference to pp. 257, 282 

 and 283 of this journal will 

 show this clearly, if columns a, 

 d and g are compared ; we do not, 

 however, wish to enter into this 

 question at present, but will endeavor 

 to show how penetration may be 

 given to wide angle lenses. 



It seems upon inquiry that some 

 of our opticians have for some time 

 past produced a half-inch objective 

 for use with the binocular, by placing 

 a diaphragm of smaller aperture than 

 usual behind the back lens of the 

 objective. The half inch of 60° is 

 thus easily reduced to 40°, and the 

 penetration consequent upon such re- 

 duction is by this means obtained. 



For some time past we had been us- 

 ing stops of blackened card-board, and 

 as these were not very convenient 

 we had a conversation with Mr. 

 J. B. Dancer of Manchester, as to the 

 utility of making an "iris " diaphragm 

 for the purpose, when he produced a 

 graduating diaphragm made ten years 

 ago, but to the best of our know- 

 ledge, our idea has never been pub- 

 lished. 



It will be seen that this form of 

 aperture shutter enables the operator 

 to adjust his objective to any aperture 

 he wishes, and this cannot be effected 

 upon any older plan without having 

 a large set of stops on hand. The 

 closing of the shutter does not con- 

 tract the absolute size of the field, but 

 only the brightness of it, and the true 

 value oi penetration can easily be ob- 

 served without moving the eye from 

 the tube. 



The value of wide apertures for 

 good definition may also be seen when 

 using this "shutter." If Topping's 



admirable preparation of the proboscis 

 of the blow-fly be observed with an 

 inch objective having an air angle of 

 30°, the view is superb, the pseudo- 

 tracheal markings come out well de- 

 fined and sharp ; but close the shut- 

 ter until an angle of 14° or less is ob- 

 tained and examine again, when it 

 will be found that the definition is 

 not nearly so good, while there is 

 more penetration, the whole of the 

 pseudo-tracheal tube being observed 

 under one focussing. While in this 

 condition, the eye being still applied 

 to the tube, open the shutter to its 

 full extent, and the effect of wide 

 aperture will demonstrate itself. 



Perhaps the best object to show 

 the amount of penetration possessed 

 by objectives of low angle, may be 

 found in the microfungus Myxotri- 

 chum defiexum, or M. chartarum ob- 

 served under the one-inch objective. 

 The former object consists of little 

 patches of grey downy balls, from 

 which arise a number of radiating 

 threads furnished with a few opposite 

 and deflexed branches. Under an 

 inch objective of 30° air angle but 

 few of these branches can be seen 

 under one focussing, the remainder 

 being enveloped in a haze of light ; 

 but if a central layer be focussed, the 

 simple closing of the shutter will suf- 

 fice to bring the superior and inferior 

 layers into view, though of course 

 the image is not so bright or so well 

 defined as before. 



The objection to wide angles, that 

 they do not possess penetration may 

 now be fairly said to have broken 

 down ; its other phase, that of work- 

 ing-distance, we will treat of in a 

 future number. 



Staining of Living Unicellular 

 Organisms.* 



K. Brandt finds haematoxylin and 

 Bismarck-brown suitable coloring ma- 

 terials with which to stain Protozoa in 

 the living state. For Amoebae and 

 Heliozoa a dilute solution of haema- 



♦Abstract, in Joum. Royal Micros. Society. 



