140 Mr Newall, On a combination [Jan. 29, 



combination but also the light reflected from the surface AB 'a.t 

 primitive incidence. The reflected light gives rise to a simple 

 image of the slit, which appears to be superposed upon the 

 spectrum ; for ease in description and for an obvious reason this 

 will be called the " pointer." If the prism-combination is turned 

 through a small angle hd, the pointer moves through the angle 

 2S^ but the spectrum moves through a much smaller angle, 

 whose magnitude varies with the part of the spectrum considered. 

 Hence the pointer can be made to coincide with any line in the 

 spectrum and its change of position is known in terms of the 

 corresponding change of position of the prism-combination. If 

 therefore a suitable micrometer movement is used to move 

 the prisms, the position of the pointer may be read off the 

 micrometer. 



The line in the spectrum which coincides with the pointer is 

 always that which is due to rays which have passed symmetrically 

 through the prism combination. The movement of the prism gives 

 symmetrical passage to different rays in turn and the pointer 

 indicates which ray has passed symmetrically. 



The course of rays which pass symmetrically through the 

 prism -combination is shown in the figure ; such rays emerge from 

 the face AG in a direction parallel to the plane of symmetry, and 

 consequently the deviation in passing through the prism A is 

 equal to the angle of primitive incidence : 



JD = ^ = (j) + yjr -A, 



whence A ^'yfr, or the angle of emergence from the face AC is 

 equal to the angle A. The angle of emergence can be made, 

 under readily calculable conditions, equal to the angle of primitive 

 incidence, but generally only with rigid accuracy for rays of one 

 refrangibility ; hence the rays which pass symmetrically will not 

 in general pass with absolute minimum deviation. 



The 'pointer' may be considered as being connected with the 

 prism, and independent of the observing telescope. It is thus 

 attached, so to speak, to the strongest part of the instrument 

 instead of the weakest, where the micrometer is usually placed, 

 namely the eye end of the telescope. The telescope is used 

 merely as a magnifier. The need for carefully worked surfaces 

 for the prisms forms perhaps the strongest objection to the use of 

 this combination ; curvature in any one surface of either prism 

 must throw the points and the spectrum into different focal 

 planes in the observing telescope and so introduce parallax diffi- 

 culties which can only be eliminated by reworking the faulty 

 surface. Two prisms which I have in my possession and which 

 were worked by Hilger, have been used to test the capabilities of 

 the combination, and give excellent results. 



