428 Prof. D. B. Brace on a New Spectrophotometer 



rotation of R. This last may be omitted and a micrometer 

 screw with graduated head used, the ring Q being- dis- 

 pensed with entirely. This telescope is also provided with a 

 variable ocular slit over which may slide the ocular E. The 

 compound prism P is mounted on a plate above the axis of 

 the instrument which passes through the prism at the inter- 

 section of the axis of the telescope with a line drawn perpen- 

 dicular to this axis from the vertex of the prism next to T'. 

 which is the radiant axis of colour. This plate has a slight 

 movement at right angles to the axis, for purposes of adjust- 

 ment, and is capable of being screwed permanently to the 

 body of the instrument. The axis of the telescope should also 

 pass through a point in the adjacent face of the prism at half 

 its height and one-fourth the distance across the face from 

 the vertical angle formed by the two half-prisms, to the angle 

 adjacent to T x . The axis of each of the collimators should 

 lie at the same height as the axis of the telescope, and should 

 intersect the back face half-way between the adjacent vertical 

 angle and the dividing plane of the prism. Each half of 

 this back face then receives, symmetrically, the rays from 

 the collimator to which it is adjacent ; and the telescope 

 receives the combined rays from that half of the front face to 

 which it is adjacent. The silvered strip between the two half- 

 prisms may be seen within the prism P. This strip is deposited 

 upon the right half of the compound prism, so that reflexion 

 takes place on the side next to the glass upon which it is 

 deposited, so as to insure a better surface and less oxidation. 

 It is essential that the lines bounding this strip be sharp and 

 parallel to each other, and in a plane normal to the refracting 

 edge of the prism which is adjacent to T x , in order to avoid 

 diffraction. 



The total-reflecting prisms are mounted on the slits of T 

 and T', so that the instrument may be used for spectroscopic 

 work and also as a modified form of Crova's or of Vierordt's 

 spectrophotometer, by inserting the eyepiece E and examining 

 the two contiguous spectra either from T and T x , or from one 

 of them, using the total-reflecting prism. 



With this instrument the setting for minimum deviation for 

 sodium light is first made with the collimator T. The prism 

 P is then fastened permanently, and the telescope R clamped, 

 and the collimator T' then adjusted until the image of the slit 

 is bisected by the ci'oss-hairs. T' is then clamped permanently. 

 The divided arc M may then be adjusted to zero, correspond- 

 ing to the sodium line. It is then calibrated for the different 

 Frauenhofer lines or for different wave-lengths of light. The 

 eyepiece E is then removed, and the ocular slit adjusted so as 



