412 Mr. R. W. Cheshire on a New Method of Measuring 



various parts of the apparatus are mounted rigidly in any 

 convenient manner, and the two edges and the observing 

 telescope B. should be approximately centred with the axis 

 of the principal lens C. The observing telescope, situated 

 at a distance of roughly 5 metres from the lens C, has an 

 aperture of about 3*5 cm. and a magnifying power — in 

 normal adjustment — of x 24. The edge F is secured to a 

 rack-and-pinion motion providing for a slow transverse 

 movement across the front of the observing telescope in a 

 right- or left-hand direction. The desired parallelism of the 

 two edges may be checked either by removing the eyepiece 

 of the observing telescope and looking down the tube, or by 

 examining with a small pocket magnifier the images of the 

 two edges projected into the Ramsden circle of the telescope. 

 A small rectangular glass cell, E (see below), containing the 

 immersion fluid and the glass under test, K, is adjusted in 

 the usual way on the prism, D, of the refractometer (not 

 shown in diagram), which is placed just in front of the 

 lens C and at such a height that the cell is approximately 

 opposite the centre of the lens. Suppose, for the sake of 

 simplicity, that a prism of small angle is being examined 

 and that it is immersed in the fluid contained in the cell 

 with its refracting edge vertical. In general the refractive 

 indices of the glass prism and the surrounding liquid will 

 differ, and the glass will therefore exert a prismatic effect 

 upon light passing through it. The imaue of the edge B 

 will therefore present a doubled appearance, the image 

 formed by light passing through the glass being displaced 

 laterally by a small amount relatively to the rest of the 

 image depending upon the magnitude of the difference 

 between the two indices. Consequently, on traversing 

 slowly the edge F across the objective of the observing 

 telescope, which will be supposed to be focussed upon the 

 prism, if will be found that the surface of the prism gradually 

 darkens and finally becomes completely black, whilst the 

 side faces and the rest of the cell will show up brightly. 

 On continuing the steady motion of the edge F these faces 

 also will begin to darken, until finally no light passes into 

 the observing telescope and the whole field appears black. 

 The order of these two phenomena is determined by the sign 

 of the difference of the refractive indices of the fluid and 

 prism. If now the refractive index of the immersion fluid 

 be changed somewhat, it will be at once obvious, from she 

 relative interval at which these phenomena occur, whether 

 the difference in the refractive indices has been increased or 

 decreased by the alteration. In the course of a few trials it 



