274 SUMMARY OF CURRENT RESEARCHES RELATING TO 



" By the above remarks," Dr. EoUet says, " I imagine I have 

 sufficiently shown that the iise of the spectroscopic eye-piece in 

 microscopy is somewhat limited, but if it be objected to this that 

 Sorby has made exceedingly numerous observations by this means, it 

 may be replied that these researches were concerned only with the 

 discovery of the characteristic absorption-spectra of colouring matters, 

 that is, always for the solution of a particular problem for which this 

 eye-piece is eminently fitted. There are, however, a large number 

 of problems in micro-spectroscopic research for which the eye-piece 

 is not suitable, i. e. all those in which it is not merely required to 

 examine the absorption-spectra of the colouring matters occurring in 

 microscopical objects, but the objects themselves in monochromatic 

 light, whether in any particular part or all parts of the spectrum. For 

 such purposes the spectrum projected in the plane of the object must 

 be used as it was employed before the introduction of the more 

 recent eye-piece." 



The following description is then given of the polari-spectro- 

 microscope which was constructed by Schmidt and Haensch of Berlin, 

 according to Dr. EoUet's directions. 



To a Microscope (fig. 43) in which the stage is further than 

 usual from the base, the following pieces of accessory apparatus are 

 fixed. 



I. Beneath the stage t, is a small spectroscope h attached to it 

 by means of a metal plate a with an oval hole, and movable by the 

 screw c horizontally from right to left in a slide applied to the 

 metal plate. 



The spectroscope consists of the following parts (fig. 44): — 

 (1) The slit s adjusted by the screw d; (2) a collimator lens e; (3) a 

 direct- vision prism /; and (4) above the prism a convex lens of short 

 focus g, which is intended to project a small spectrum in the plane of 

 the object on the stage. That this may be easily done with different 

 objects, on slides of diff'erent thicknesses, the prism and the convex 

 lens can be moved vertically, they being in one piece of tubing, while 

 the slit and collimator lens are in another. This movement is 

 efiected by a screw h cut in the inner tube, and a ring i (figs. 43 and 

 44) on the outer, which act like the correction-adjustment of 

 objectives. (The amount of the vertical movement can be registered 

 on a millimetre scale, divisions on the ring showing fractions of 

 mm.) The dispersion of the prism is such that with a medium 

 magnifying power, the small spectrum projected in the plane of the 

 object, can be completely seen in the field of the Microscope, from 

 the red to the violet end and the Fraunhofer lines also clearly visible. 

 (5) In front of the slit is a polarizing (Hartnack-Prazmowski) prism 

 A;, and (6) above the convex-lens is fixed a selenite film e (Eed I. Ord. 

 or Eed II. Ord.). 



II. Over the tube of the Microscope is an eye-piece o (fig, 48), 

 above which a Hartnack-Prazmowski analysing prism is fixed. This 

 is movable over the eye-piece by its tube p and its correct position 

 is shown by an index q on the tube moving over a circular scale r 

 fixed to the eye-piece. 



