1883.] 



MICROSCOPICAL JOURNAL. 



169 



tect the slightest property of double 

 refraction. For this purpose a polar- 

 izing prism X', {f\g. 28) is added, and 

 a selenite plate c is placed above the 

 lens g, giving a red color when the 

 analizer is used, which is placed in 

 the ocular with the ordinary gradu- 

 ated scale r, and index (/. 



When light is thrown into the in- 

 strument from the mirror it is first 

 polarized by the prism k, the ordinary 

 ray passing through the slit s and the 

 lens <?, is spread out into a spectrum 

 by the prisms /, and focussed upon 

 the object by the lens g. But before 

 it reaches the object it has to pass 

 through the plate of selenite r. Here 

 its plane of polarization is changed. 

 In an ordinary polariscope one part of 

 the ray is retarded more than another 

 part, so that when the light from the 

 selenite reaches the analizer, the two 

 portions become partially reunited, 

 and produce color. But here we have 

 another phenomenon which is quite 

 too complex to be explained in this 

 place, for we have to deal with the 

 effect of the selenite upon the light 

 decomposed by the spectroscopic 

 prism. The most that can be said is, 

 that when the apparatus is arranged 

 according to the plan shown in fig. 

 29, there will be a dark interference- 

 band in the spectrum, the position of 

 which will depend upon the thickness 

 of the selenite tilm. In this figure, ss 

 represents the direction of the slit of 

 the spectroscope, />/>, the plane of 

 vibration of the polarized ray, a a the 

 plane of vibration of the analizer, and 

 00 the plane of vibration of the ordi- 

 nary ray in the selenite film. 



Since the position of the narrow in- 

 terference-band in the spectrum de- 

 pends upon the thickness of the sele- 

 nite film, it follows that by placing 

 another thin film above the plate, 

 either resting upon it or upon the 

 stage of the microscope, with its opti- 

 cal axis coinciding with the first film, 

 the dark band, which we will suppose 

 to be in the yellow of the spectrum, 

 moves away from that place, and the 

 yellow light passes through. Now, 



anv doubly refracting object will act 

 the same as the additional film of se- 

 lenite, so that if we ex'amine a piece 

 of muscle, for example, which has been 

 found to possess the power of double 

 refraction, the effect will be the same 

 as increasing the thickness of the sel- 

 enite ; and the bit of muscle, being 

 smaller than the dark band, will be- 

 come luminous upon a dark ground. 



To use the instrument, the object 

 to be examined is placed in the dark 

 interference-band. If it has no power 

 of double refraction, it will, in all azi- 

 muths, appear dark, but if double re- 

 fracting it will, in certain azimuths, 

 appear bright on a dark field. 



Mounts and Mounting. 



Continued from page 156. 



Asparagine, another choice crys- 

 tallization, must .not be omitted. 

 The finest results are obtained by 

 making a hot saturated solution in 

 water, of which solution a drop must 

 be evaporated on the centre of the 

 slide, by means of very gentle heat. 

 The slide must be removed from the 

 inll'-;ence cf heat as soon as an 

 amphoicvF. P.lni replaces the solution, 

 and as the slide cools the crystals 

 will appear. Another form approach- 

 ing closely that of wheel or circular 

 crystals, described earlier in my paper, 

 may be obtained by having the solu- 

 tion of less strength. 



It has probably been noticed that 

 the crystals hitherto described have all 

 been obtained from aqueous or al- 

 coholic solutions, and it is a fact that 

 in each case, although I have ex- 

 perimented with other solvents, as 

 ether and chloroform, the best results 

 have been obtained in the manner 

 described ; but I cannot leave this part 

 of my subject without mentioning two 

 crystallizations (those of salicine and 

 santonine) of which the best solution 

 can be obtained by methylated chloro- 

 form ; but even in these two cases 

 solutions Ao not give the best results, 

 and it is noticable that in all, or nearly 



