272 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The stage is intended to be used with transparent objects. The 

 central aperture receives either a wheel of diaphragms or a glass 

 disk. An erecting prism is shown in place over the eye-piece. 



"When opaque objects are required to be observed they are placed on 

 the base-plate, the plate carrying the two pillars, mirror, and stage being 

 then removed by loosening the two clamp screws at the corners. 



Rollet's Polari-spectro-microscope.* — This instriiment was de- 

 vised by Dr. A. Eollet, of Graz, and is a combination of a compound 

 Microscope with a sj^ectral and polarizing apparatus, he having 

 observed, whilst experimenting on the spectra of the colours of thin 

 plates, and the polarization colours of selenite films, that such a 

 combination might be exceedingly useful for certain histological 

 examinations. 



The description of the instrument and its use is prefaced by some 

 remarks on the spectroscopic eye-pieces hitherto designed, beginning 

 with the original plan by which parts of a spectrum (or a small 

 spectrum suiting the field) were projected in the plane of the micro- 

 scopical object. This was eflected by spectral apparatus fixed in 

 front of the objective, and thus observations could be made on the 

 behaviour of microscopical objects in monochromatic light. 



Later the spectroscopic eye-piece was adopted, on the suggestion 

 of Dr. W. Huggius, on the model of a star spectroscope, and after- 

 wards improved by the spectroscopic eye-pieces, especially adapted 

 for the Microscope, of Sorby and Browning, Zeiss, and others. 



Each of these methods, however, serves difierent purposes; and 

 careful consideration shows that it is only the older manner of examina- 

 tion which is adapted for true microscopical studies of a more extended 

 ^plication, the use of the spectroscoi)ic eye-piece being much more 

 circumscribed. In the latter the slit is at the point where the 

 inverted image is formed by the objective and field-lens. A linear 

 strip of this image is then spectrally analysed by a direct vision 

 prism. Such an a2)paratus is excellently adapted for studying the 

 absorption- spectra of uniformly coloured microscopical objects con- 

 taining no inner contours, and whose images cover the slit either 

 entirely or to a definite extent. It can also be used for the same 

 purpose in the case of the absorption-spectrum of 07ie particular 

 absorbing substance which is associated with delicate bodies uniformly 

 distributed in a liquid, as with the red blood-corpuscles or chlorophyll- 

 grains. But in these cases the action of the eye-piece is satisfactory 

 only if the object is somewhat above or below the focus of the Micro- 

 scope. It is easy to see the reason for this, but an examj^le will explain 

 it more clearly. Place a drop of blood, spread out on a slide, under 

 the Microscope, remove the prism of the eye-piece, and with the slit 

 wide open focus so that the image of the blood-corpuscles may be as 

 sharp as possible, then narrow the slit and replace the prism. A 

 spectrum of unequal brightness will be seen, crossed by numerous 

 dark lines and shadows at right angles to the direction of the slit, 

 and in which both the Frauuhofer lines and the absorption-bands 



* Zeitschr. f. luslrumenteuk., i. (1881) pp. 366-72 (3 figs.). 



