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 Prism 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) .
