65G 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



by their firm. The glasses are arranged in order of descending values 

 of v. Those whose factory number is preceded by the letter A are the 

 ordinary silicate crowns and flints which have been in use for over half 

 a century. Those marked with the prefix B are of more modern intro- 

 duction, while those preceded by C are of quite recent introduction. 



The optical constants as given in the table are to be regarded as 

 type values, which are adhered to with considerable accuracy from one 

 melting to another. The spectrum lines used for the specification of 

 these constants are the lines of the hydrogen spectrum known as C, 

 F and G', and the sodium line D to which latter the refractive index 

 n D refers. The wave-lengths of these lines may be taken in micro- 

 millimetres as follows : — 



C = 0-6563 

 F = 0-4862 



D = 0-5893 

 G'= 0-4341 



The difference between the refractive indices for the C and F lines, 

 generally called the interval C — F, is defined as the mean dispersion, 

 while the partial dispersions and their relative values, obtained by 

 dividing the partial dispersion by the mean dispersion, are also specified. 

 The value of v is given by 



1 



v = n. 



C-F. 



Chance Brothers' Cover Glasses of thin Glass for Microscopic 

 Preparations.* — This thin glass is made in three thicknesses, and in all 

 usual sizes both square and round ; larger pieces for special purposes are 

 also supplied. This glass is chemically of the " hard crown " type, but 

 differs in its mode of manufacture. Its optical constants, which have 

 been measured by means of specially prepared prisms, are as follows : — 



Manipulation of the Microscope.!— This most excellent manual, 

 the work of Edward Bausch, was originally published twenty years ago, 

 since when it has deservedly run through four editions. In simple 

 language are described the stand, its various parts and accessories, how 

 to manipulate these in the proper way and with the best effect, the 

 volume ending with instructions as to the care of a Microscope. The 

 index is quite complete. 



Elementary Microscopy 4 — This handbook on Elementary Micro- 

 scopy is the outcome of a series of articles on "Microscopy for 

 Beginners," by F. Shillington Scales. The material has been re-cast 



* Catalogue, Optical Convention, 1905, p. 4. 



t Bausch and Lomb Optical Co., Rochester, N.Y., 4th ed., 1901, 202 pp., with 

 numerous illustrations. 



t London: liailliere, Tindal and Cox, 1905, xii. and 179 pp., 77 figs. 



