230 SUMMARY OF CURKENT RESEARCHES RELATING TO 



crystalline face and gives a good knife-edge. Grayson finds the stones 

 from the diamondiferous drift of New South Wales best for this purpose, 

 and when ruling properly such a diamond makes no noise. The photo- 

 graphs which accompany the paper show that the rulings are extremely 

 regular, and warrant the hope that gratings ruled on the machine will 

 give exceptionally clear spectra. The verdict of spectroscopists on the 

 gratings will be awaited with considerable interest. In the meantime 

 all will congratulate Grayson on the completion of his work, and the 

 University of Melbourne on the public-spirited way in which it has 

 provided facilities for that work. 



Dispersion and other Optical Properties of Carborundum."' — H. E. 



Merwin's investigations on carborundum show that for wave-lengths 

 ranging from 755 /a/a to 410 /x/a, w ranges from 2*616 and c from 2 '654 

 to 2*812. Microscopical study of several samples of granular carbo- 

 rundum revealed no definite variations in the refractive index for red 

 light, even in grains of different colour. 



Measurement of Magnifying Power.f — W. M. Bale describes a 

 simple method of measuring the magnifying power of a microscopical 

 combination. His procedure is : — (A) Measure with the camera lucida 

 the exact diameter (at 10 inches) of the magnified field ; (B) measure 

 with the stage micrometer the actual diameter of the field. Then A 

 divided by B is the magnifying power. At the same time there are 

 sundry precautions to be taken to ensure success, especially if the investi- 

 gation does not concern the centre of the field. These difficulties the 

 mithor fully deals with. 



(6) Miscellaneous. 



Balsam Problem. J — At a Meeting of the Optical Society on April 11 

 J. W. French stated that for cementing optical parts together Canada 

 balsam is almost invariably employed. Although on starting or starring 

 of the balsam-layer actual separation of the parts or deformation of 

 the optical surfaces frequently occurs, there is no appreciably better 

 substance known. Optical parts may be combined with an air-space 

 between the surfaces by optical contact, with or without scaled edges, by 

 •optical welding or by cementing. The disadvantages of the various 

 methods were enumerated, the loss of light at transmission surfaces being 

 particularly discussed. A considerable number of lialsam specimens, of 

 ages varying up to ten years, had been opened, and photomicrographs of 

 the balsam-layer were exhibited. In all cases there were fluid layers 

 between the harder balsam and the glass surface, and the photographs 

 demonstrated particularly the smallness of the adhesion to the glass. 

 Specimens artificially produced were also exhibited. In many cases the 

 age of the specimen was shown to be deducible from the configuration. 

 So-called granulation of balsam was stated to be due to the action of 

 moisture on the balsam surface. No trace of crystallization of glass- 



* Journ. Washington Acad. Sci., vii. No. 14 (1917) pp. 445-7. 

 t Journ. Quekett Micr. Club, xiii. (1917) pp. 1-8 (1 fig.). 

 X Nature, April 18, 1918, p. 139. 



