122 SUMMARY OF CURRENT RESEARCHES RELATING TO 



plates, by which the upper plate can be brought nearer to or further 

 from the lower plate. The result of this is to make the stage more or 

 less sensitive. If, for instance, the plates are widely apart so that the 

 pair of springs between them are relaxed, the upper plate yields to the 

 slightest touch ; when, however, the plates are brought closer together, 

 so that the springs are compressed, the upper plate is much more rigid. 



Engelmann's Microspectrometer.* — Prof. T. W. Engelmann points 

 out that both the microscopic anatomist and physiologist are com- 

 pelled to use peculiar methods of research, and that this is especially the 

 case when it is necessary to examine properties and appearances quanti- 

 tatively as well as qualitatively. A review of the ordinary methods of 

 microscopical investigation shows that they are almost solely qualitative. 

 As a contribution therefore to quantitative methods of microphysio- 

 logical research, the author describes a microspectrometer for the analysis 

 of the colour of microscopically small objects. 



Originally devised for the quantitative determination of the absorp- 

 tion of different colours through living plant-cells, the apparatus is 

 serviceable for quantitative microspectral analysis generally, and can be 

 used with advantage for most microspectrometrical researches in place 

 of the ordinary larger apparatus. The principle of the instrument is 

 practically that of Vierordt's spectrophotometer. The spectrum of the 

 object is compared with a standard spectrum, and quantitative measure- 

 ments are obtained by altering the width of the slit until the brightness 

 of corresponding parts of the two spectra is the same. The apparatus, 

 which in use takes the place of the eye-piece in the body- tube, is repre- 

 sented in figs. 14, 15, and 16. The lower part contains the two slits and 

 the arrangement for obtaining a comparison spectrum. The upper part 

 is the spectroscope proper. 



The under part consists of a rectangular box A, provided above and 

 below with wide circular openings, into which are screwed the tubes h 

 and c. The tube h fits in the place of the ordinary eye-piece, and is 

 fastened by the screw b', while into the tube c fits the eye-piece oc 

 during the setting up of the object, replaced later by the cylindrical 

 underpiece a' of the spectroscope. The latter rests with the ring r 

 in the circular groove s, and is here fixed in a constant position with 

 respect to the slits. The insertion and removal of the upper piece can 

 thus take place without any shaking, so that there is much less danger 

 of displacement of the images than in the micro-spectral ocular of Abbe 

 and Zeiss, in which the two pieces are movable one within the other. 



In the right of the box A is fixed the small tube d, through which, 

 by means of a mirror S or lens, the light from a source at the side is 

 directed upon the totally reflecting prism pr. By means of the handle 

 h projecting from the box A at h', this prism can be brought at will 

 beneath or out of reach of the right slit which gives the standard 

 spectrum. 



The tube d carries at its end a frame n for the reception of diaphragms 

 and ground or coloured glasses. In order in all cases to obtain a uniform, 

 and, from the position of the observer's eye, as far as possible indepen- 

 dent illumination of the standard slit, at the recommendation of Prof. 

 Abbe a lens is fitted into the inner opening of the tube d ; this throws a 



* Zeitsclir. f. Wiss. Mikr., v. (1888) pp. 289-9o (1 fig. and 1 pi.); and Arch. 

 NeeiluncL, xxiii. (1888) pp. 82-92 (I fig. and 1 pi.). 



