804 SUMMARY OF CUERENT RESEARCHES RELATING TO 



of thick indiarubber cemented between tbem on three sides, the 

 fourth side being left open, and thus forming a very convenient flat 

 bottle for the side coat-pocket. The space between the glasses is 

 sufficient to allow of Anacharis 5 in. long being inserted without 

 pressure, at the same time enabling the collector to bring all parts of 

 the weed into good focus. By the insertion of an indiarubber flat 

 cork the bottle is rendered water-tight, and can be used as a slide on 

 the stage, so as to obviate the necessity of disturbing the weed should 

 any object of interest be observed when collecting. 



Mr. Hardy also proposes a simple and effective method of straining 

 the water poured into or out of an ordinary wide-mouthed collecting 

 bottle, viz. by means of a small cylinder of copper wire gauze, which 

 extends above the neck of the bottle. 



Eye-piece Amplification. — Prof. Abbe points out that his view 

 as to the comparatively low eye-pieces which the best Microscope 

 objectives of the present day will usefully bear * is supported by the 

 recognized rules for telescopes. 



" The essential principle for a valid comparison of the telescope 

 and the Microscope is that every Microscope involves in its action 

 that of a given telescope. The effect of the Microscope cannot 

 in any case extend farther than the effectiveness of such telescope. 

 Now the most trustworthy power of eye-piece for a telescope is 

 approximately 40 per inch of the diameter of the objective, i. e. 

 1/4 in. focal length for every telescope in which the proportion of 

 focal length to aperture is 1 : 10. This relation of eye-piece to 

 objective in the telescope is exactly paralleled in the Microscope 

 when to a 1/8 in. dry objective of maximum aperture is applied (with 

 a 10-in. tube) a 1 in. eye-piece, or a 3/4 in. eye-piece with a homo- 

 geneous immersion 1/8 in. of 1 • 33 N.A. 



If therefore it is contended that Microscope objectives can 

 usefully bear the application of a 1/4 in. eye-piece, it must at the 

 same time be contended that a telescope will bear a useful power of 

 120 per in. aperture !" 



lUumination and Focusing in Photo-Micrography. f — Dr. E. A. 

 Hayes, after considerable experience with electric (arc) magnesium, 

 lime, gas, and oil-lamp lights, finds that only the lime-light and the 

 oil-lamp fulfil the necessary conditions required in the case of a 

 source of artificial light for photo-micrography which shall at the 

 same time have light-illuminating power, be perfectly steady, possess 

 very active actinic properties, and be easily produced and maintained. 

 The use of the oil-lamp being confined to cases where the magnifying 

 power does not exceed 50-100 diameters ; or in other words, to the 

 1 in. or 1/2 in. objective. The difficulty as to the intensity of the 

 light is not so much in reference to the exposure of the plate, as to 



* " Usefully " that is in the sense defined in Prof. Abbe's paper, Vol. III. 

 (1883) p. 790, and not merely useful" for an amusing exhibition of the diffrac- 

 tion phenomena. 



t Proc. R, Irish Acad. (Sci. iv. (1884) pp. 59-61. 



