62 MICRO-SPECTROSCOPE AND POLAR/SCOPE. 



consequently there are no clearly defined dark bands, the light is 

 simply less brilliant in certain regions and the red rays so preponder- 

 ate that they give the prevailing color. 



§ 148. Absorption Spectrum of Colored Minerals. — As example 

 take some malazeit sand on a slide and either mount it in balsam (see 

 § 176), or cover and add a drop of water. The examination may be 

 made also with the dry sand, but it is less satisfactory. Light well with 

 transmitted light, and move the preparation slowly around. Absorp- 

 tion bands will appear occasionally. Swing the prism-tube off the oc- 

 ular, open the slit and focus the sand. Get the image of one or more 

 grains directly in the slit, then narrow and shorten the slit so that no 

 light can reach the spectroscope that has not traversed the grain of 

 sand. The spectrum will be very satisfactory under such conditions. 

 It is frequently of great service in determining the character of unknown 

 mineral sands to compare their spectra with known minerals. If the 

 absorption bands are identical, it is strong evidence in favor of the iden- 

 tity of the minerals. 



REFERENCES TO THE MICRO-SPECTROSCOPE AND SPECTRUM ANALYSIS. 



§ 149. The micro-spectroscope is playing an ever increasingly important role in 

 the spectrum analysis of animal and vegetable pigments, and of colored mineral 

 and chemical substances, therefore a somewhat extended reference to literature 

 will be given. Full titles of the books and periodicals will be found in the Bibli- 

 ography at the end. 



Angstrom, Recherches sur le spectre solaire, etc. Also various papers in peri- 

 odicals. See Royal Soc's Cat'l Scientific Papers ; Anthony & Brackett ; Beale, p. 

 269 ; Behrens, p. 139 ; Behreus, Kossel und Schiefferdecker, p. 63 ; Carpenter p. 

 104 ; Browning, How to Work with the Spectroscope, and in Monthly Micr. Jour., 

 II, p. 65 ; Daniell, Principles of Physics. The general principles of spectrum 

 analysis are especially well stated in this work, pp. 435-455 ; Dippel, p. 277 ; Frey ; 

 Gamgee, p. 91 ; Halliburton ; Hogg, p. 122 ; also in Monthly Micr. Jour., Vol. II, 

 on colors of flowers ; Jour. Roy. Micr. Soc, 1880, 1S83 and in various other vols. ; 

 Kraus ; Lockyer ; M'Kendrick ; Macmunn ; and also in Philos. Trans. R. S., 18S6 ; 

 Various vols, of Jour. Physiol.; Nageli und Schwendener ; Proctor; Ref. Hand- 

 Book Med. Sciences, Vol. I, p. 577, VI, p. 516, VII, p. 426 ; Roscoe ; Schellen ; 

 Sorby, in Beale, p. 269, also Proc. R. S., 1874, p. 31, 1867, p. 433 ; see also in the 

 Scientific Review, Vol. V, p. 66, Vol. II, p. 419. The larger works on Physiology, 

 Chemistry and Physics may also be consulted with profit. 



MICRO-POLARISCOPE. 



\ 150. The micro-polariscope or polarizer is a polariscope used in connection 

 with a microscope. 



The most common and typical form consists of two Nicol prisms, that is two 

 somewhat elongated rhombs of Iceland spar cut obliquely and cemented together 

 with Canada balsam. These Nicol prisms are then mounted in such a way that 



