66 



THE AMEKICAN MO:^THLY 



[April, 



Now take the case of a high-power, 

 say a ^-inch. The field of view {a b 

 in Fig. 20,) let us suppose, is i™™' in 

 A & K 



Fig. 20. 

 diameter ; the thickness of slide, or 

 distance of diaphragm below the ob- 

 ject, the same ; the diaphragm -aper- 

 ture h {, .5°™- (For simplicity's sake, 

 the effect of refraction of the slide 

 is not shown.) A pencil of light of 

 about 30° is then admitted, axially, 

 to that part of the object in the centre 

 of the field ; but those which lie near 

 the margin are illuminated only by 

 oblique rays (either from the edge of 

 the mirror or outside luminous objects, 

 or both) as is perfectly evident from 

 the diagram : and suppose we have a 

 valve of P. angulatum lying near the 

 margin of the field, it will very likely 

 be resolved by this mode of obtaining 

 strictly central illumination. If a lar- 

 ger aperture is used, extraneous 

 rays will be admitted, and the defini- 

 tion correspondingly marred. But 



jlGB 



Fig. 21. 

 suppose we obtain an illuminating 

 pencil of rays of the same angle as 

 before (30°) by placing a larger dia- 

 phragm at//nn Fig. 21 ; approximately 

 central light is obtained for all parts 

 of the field, and outside rays are ex- 

 cluded from the object and object- 

 glass. 



Diaphragms the size of the field, 



placed immediately above or imme- 

 diately below the slide, may be of 

 use to exclude rays from the marginal 

 portion of an object-glass that can- 

 not be brought to a focus within the 

 field, and which consequently do no 

 good but rather harm by causing re- 

 flections from the various lens-sur- 

 faces of the object-glass. Inthis case, 

 such diaphragms would, no doubt, be 

 beneficial ; but they would have this 

 function only, except in the case of 

 the highest powers, and would not 

 obviate the necessity for others placed 

 at a suitable distance below. The 



li M. K" ti 



Fig. 22. 



diagram (Fig. 22) shows the diaphragm 

 h i in place just beneath the object ; 

 the rays e k, e m, e /, f k, f n, f I, which 

 could not form any image in the eye- 

 piece, are prevented from entering 

 the object-glass. 



It therefore appears that each sort 

 of diaphragm has its own particular 

 function or functions ; of which those 

 possessed by the diaphragms at a 

 distance below the object are the 

 more important. But the perfect mi- 

 croscope must have both. 



Philadelphia, Feb. 25th, 1881. 



o 



Motion of Diatoms. 



BY J. D. COX. 



The investigation of the motion of 

 diatoms is occupying the attention of 

 observers in Europe, and a Russian 

 naturalist has recently published 

 some experiments made with infu- 



