ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



327 



Nelson's Simple Condenser.* — E. M. Nelson's object in sug- 

 gesting this condenser was to provide a very inexpensive, but at 

 the same time efficacious means of illumination for the micro- 

 organisms which are now the subject of so much investigation, and for 

 which purpose it is desirable that the necessary apparatus should 

 involve as little outlay as possible. As exhibited it was attached to a 

 Leitz 3/. 12s. instrument. 



Fig. 79. 



It consists of a socket A, having a spiral slot. This socket screws 

 into the stage, and in it slides a second tube B which has a 1^ in. 

 milled flange for focusing by rotation, and a pin which works up and 

 down in the slot. This tube carries the lenses, which are a meniscus 

 and biconvex. A cap C with large aperture in the centre fits at the 

 end of the tube to hold diaphragms, like that shown at D. The 

 small vertical piece of the slot prevents the condenser being acci- 

 dentally twisted out. The aperture is • 5 N. A. 



Madan's Method of isolating Blue Rays for Optical Work. — 

 Mr. H. G. Madan finds a combination of ordinary blue glass with a 

 peculiar bluish-green glass, known as "signal-green" glass, much 

 more convenient than the usual glass cell filled with solution of 

 cuprammonium sulphate. Glass coloured with cobalt absorbs most 

 of the rays of medium refrangibUity, but transmits besides blue rays 

 a portion of the red rays in the neighbourhood of Fraunhofer's line A. 



The " signal-green " glass (so called from its being used for rail- 

 way signal lamps) is remarkably opaque to red and yellow rays, and 

 hence if a piece of it is superposed on cobalt-blue glass, the only 

 light transmitted by both is that which lies between Fraunhofer's 

 lines F and G, constituting a beam at any rate not less homogeneous 

 than that transmitted by cuprammonium sulphate. 



In cases where the double thickness of glass may be an inconveni- 

 ence, as in disks for stage diaphragms, a plate of " flashed " blue glass 

 may be cemented, flashed side downwards, upon the signal-green 

 glass, and then the whole of the colourless part of the blue glass can 

 be ground away, leaving only the coloured film upon the signal green, 

 and thus forming a plate hardly thicker than the latter alone. 



* Engl. Mech., xli. (1885) p. 34 (1 fig.). 



