178 FRANCIS DEAS ON SPECTRA FORMED BY THE PASSAGE OF 



ing power employed. Each form of arrangement has, however, its advantages 

 as well as disadvantages, which it would be out of place to discuss here. 4 '" 



The polarising part of my apparatus consists of two Nicol's prisms, for one 

 of which, when desired, a double image prism can be substituted. 



The polarising prism is carried on the substage. It is inserted just above 

 the slit in a short tube in which it can be freely turned by a graduated head. 

 The analysing prism is placed in the usual way — in a cap over the eye piece. 



The film of selenite to be examined having first been mounted in balsam 

 between two thin glasses is placed on the stage of the microscope like an 

 ordinary object. 



It is a great convenience in this class of experiments to have the stage of the 

 microscope not only capable of rotation in the optical axis of the instrument, 

 but graduated. 



By this means we can at any time, without displacing the film under exami- 

 nation, adjust its neutral axes at any required angle to the plane of polarisation. 



With regard to the mounting of the selenite films for examination the 

 following method will be found convenient : — Make in the turning lathe several 

 wooden disks about two inches diameter and one-eighth of an inch thick. 

 Through the centre of each a hole must then be bored of about half an inch 

 diameter. A small portion round the hole is then scooped out so as to form a 

 cup, and in this the selenite is placed and secured with sealing-wax. 



The axes of the selenites are then determined and marked on the rims of 

 the clisks.t In this way any two or more selenites can be used in combination 

 with their axes set at any required angle to one another. 



It remains only to trace the course of a beam of light in passing through the 

 foregoing combination. First the ray, having been reflected from the mirror, 

 passes through the slit. It is then polarised by the first Nicol's prism, after 

 which it passes through the lenses of the achromatic condenser, and appears as 

 an image of the slit in the focus of the object glass. Having passed through 

 the selenite and the object glass, the ray enters the dispersion prisms and is 

 drawn out into a spectrum. This is magnified by the eye piece through which 

 the ray, having passed, is lastly analysed by the second Nicol's prism. 



The loss of light from the number of the above media is not so great as 

 might be supposed, still an intense source of light is desirable for satisfactory 

 results. A good artificial light placed close to the mirror will be found the best. 

 In diffused day-light rays are apt to enter the object glass by reflection from 

 the brass work without first passing through the polariser, by which the beauty 

 of the spectrum is impaired. 



* A similar form of instrument will be found described in tlie "Quarterly Journal of Science" 

 for October 1869. 



t Tbe graduated rotatoiy stage above mentioned, and wbicb is supplied by Smith and Beck, 

 affords a ready means of doing this. 



