ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 371 



(5) Microscopical Optics and Manipulation. 



Absorption Spectra of the Anilin Dyes.* — H. S. Uhler finds that 

 methyl-green absorbs the extreme ultra-violet rays, and then transmits 

 weakly in the vicinity of • 35 /a. Then a strong band with its maxi- 

 mum at 0"415/A extends from • 36-0 • 45 /a approximately. This is 

 followed by marked transparency to the green from about 0*45- 

 0"41)5 fji. Finally, beyond 0'4l)5 /a the longer waves are subjected to 

 powerful absorption, with no return even to partial transparency in 

 the visible spectrum. 



Rhodamine B has a pair of beautiful distinct absorption bands at 

 0*524 and 0'557/x. Of these, the more refrangible band has the 

 greater intensity. 



A Simple Way of Obtaining the Half-shade Field in Polarimeters.f 

 The half -shade effect in polarimeters is usually obtained either by the 

 well-known method of Laurent, or else by the more lecent method of 

 Lippich. In the former a quartz plate is employed to give the neces- 

 sary rotation to one half of the beam of polarised light propagated 

 through the instrument ; in the latter, a Nicol prism additional to the 

 polariser serves the same end. It occurred to J. R. Milne that the 

 required effect might be obtained very simply by merely interposing 

 a glass plate in the beam of light, so that half the beam traversed it 

 in the oblique direction. It follows at once, from Fresnel's laws of the 

 intensity of refracted hght, that this will produce a slight rotation of 

 the vibration-direction in the traversing half of the beam. 



The author goes fully into the mathematical theory, and gives full 

 details of the method in practice. 



Strehl, K. — Einfiihrung in die beugungstheoretische Optik. 



[A series of elaborate articles.] 



Central-Zeitung f. Opt. u. MccJl., xxviii. (1907) Nos. 1, 2, 3, etc. 



Olerici, E. — Sulla determinazione dell' indice di rifrazione al microscopio. 



Atti della reale Accadeviia dei Lined, xvi. (March 1907) 



pp. 336-43 (3 figs.). 



Cesabo, G. — Contribution a I'etude optique des cristaux en lumiere convergente. 



Acad. roy. de Belgique, Bull, de la Classe des Sci., 

 No. 5 (1906) pp. 290-34 (15 figs.). 



(6) Miscellaneous. 



Rowland's Ruling Machines. J — J. S. Ames gives an account of 

 the present condition of the three machines constructed by Professor 

 Rowland. All have been found to be more or less out of repair, but 

 two of them have been thoroughlv overhauled. Several new and im- 

 portant improvements have been added, and gratings more perfect than 

 any yet ruled can now be produced with a far less percentage of failure 

 than was formerly possible. 



Fluid Crystals.§ — J. G-. Adami and L. Aschoff record the interesting 

 observations made in their respective laboratories at Montreal and Mar- 



* Johns Hopkins Univ. Circular, No. 186 (April 1906) pp. 31-6. 



+ Proc. Roy. Soc. Edinburgh, xxvi. (1906) pp. 522-6 (2 figs.). 



X Johns Hopkins Univ. Circular No. 186 (April 1906) pp. 62-5. 



§ Proc. Roy. Soc, Series B. Ixxviii. (1906) pp. 359-68. 



