108 Messrs. V. H. Veley and J. J. Manley. 



II. The Spectrometer. 



The instrument, for the loan of which we are indebted to the 

 Government Grant Committee of the Royal Society, was obtained from 

 Becker (Meyerstein's successor) originally for the purpose of investi- 

 gating the method of determining the specific refractivities of solids 

 from their solutions.* Its graduated circle is 27 cm. diameter, 

 and reads directly to one-tenth degree ; it is furnished with two 

 reading microscopes, supplied with micrometers, three revolutions of 

 which correspond to one scale division ; as the micrometer screw heads 

 are divided into sixty parts, an observer is enabled to read directly to 

 two seconds.! 



III. The Priftm and its Fittings. 



As it was thought probable that the more concentrated acids would 

 corrode a glass prism, and the saline materials dissolved out would 

 alter the refractive indices of the samples, a prism entirely of quartz 

 was constructed under our direction by Mr. Hilger. The dimensions 

 of its plates, cut with their planes at right angles to their optic axes, 

 were 75 mm. x 68 mm. x 6 mm. diameter, of the aperture 20 mm., 

 and the angle of the prism about 60 6', an accurate measurement of 

 which is given in the sequel. At the upper end a hole was drilled so 

 that a thermometer graduated to O'l C. could be introduced, when 

 required, into the prism contents. 



Solid paraffin was selected as likely to be the best material for 

 cementing the plates on to the main block ; the paraffin was purified 

 by boiling with concentrated nitric acid, and then frequently with 

 water, until all the acid had been washed out, dried at 90 in a water 

 oven, and thus a material of constant melting point 51 obtained. 



The process of cementing was conducted as follows : The central 

 block and its plates were cleaned and the former placed upon a wood 

 slab so that the refracting angle was uppermost ; the plates were 

 placed in their proper position and the whole introduced into a water 

 oven, the temperature of which was gradually raised to 80 ; the prism 

 with the slab of wood was then lifted out and the thinnest possible 

 film of paraffin run in by capillary attraction by means of a feather 

 until the film had penetrated almost to the aperture ; the whole was 

 then replaced in the oven and allowed to cool down slowly. By this 

 process any strain due to an ixnequal rate of cooling was avoided. 



* ' Brit. Assoc. Report/ 1881, p. 155. 



t A full description of a similar instrument is given by van der Willigen (vide 

 supra) . 



