136 On Sj)ectra formed hy the passage of 



This arrangement, it will be seen, differs considerably from the 

 spectrum microscope in common use in wLicb tlie dispersion prisms 

 are placed close to the observer's eye, the- slit being in the focus of 

 the eye lens. The former arrangement has this manifest advantage, 

 that owing to the distance of the prisms from the eye, the spectrum 

 fills the whole field ; also, that the apparent breadth of the spectrum 

 can be varied at pleasure by a change of the magnifying power em- 

 ployed. Each form of arrangement has, however, its advantages as 

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



The polarizing part of my apparatus consists of two Nicol's 

 prisms, for one of which, when desired, a double-image prism can 

 be substituted. 



The polarizing prism is carried on the sub-stage. It is inserted 

 just above the slit in a short tube in which it can be freely turned 

 by a graduated head. The analyzing 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 examination, adjust its neutral axes at any required angle to 

 the plane of polarization. 



With regard to the mounting of the selenite films for examina- 

 tion 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 smaU por- 

 tion 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 disks.* 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 pass- 

 ing through the foregoing combination. First the ray, having been 

 reflected from the mirror, passes through the slit. It is then 

 polarized 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- 



* The graduated rotatory stage above mentioned, and which is supplied by 

 Smith and Beck, aflfords a ready means of doing this. 



