ON SPECTKOSCOJfIC MEASUREMENTS. 173 



this use. Fig. 1, page 171, presents tlie plan of an arrangement which, 

 while showing all the essential parts, is much Jess complicated. Starting 

 from V, a vacuum tube containing the substance whose radiations are to 

 be examined (and which is usually enclosed in a metal box in order that 

 it may be raised to any required temperature), the light is analysed by 

 one or more prisms, forming a spectrum from which any required 

 radiation may be separated from the re-st by passing through the slit S.^ 



The light from S is rendered nearly parallel by a collimating lens, and 

 then falls on a transparent film of silver on the surface of the plane 

 parallel plate Gr,.^ 



Here it divides, part being transmitted to the fixed plane mirror M, 

 and part reflected to the movable mirror M2. These mirrors return the 

 light to the silvered sui'face, where the first part is reflected and the 

 second transmitted, so that both pencils coincide on entering the observ- 

 ing telescope.^ 



A little consideration will show that this arrangement is, in all respects, 

 equivalent to a film or plate of air between two plane surfaces. The 

 interference phenomena are therefore the same as for such an air-plate. 



The theory of these interference bands has been given in an article 

 entitled Interference Phenomena in a New Form of Refractometer, ' Philo- 

 sophical Magazine ' for April 1882. As is there shown, the projections of 

 the bands are, in general, conic sections, the position of maximum dis- 

 tinctness being given by the formula 



P= -^ tan i cos2 6, 

 tan 9 



in which t^ is the thickness of the equivalent air-plate, where it is cut by 

 the axis of the telescope, 4>, the inclination of the two surfaces, 6 and i, 

 the components of the angle of incidence parallel and perpendicular re- 

 spectively to the intei'section of the surfaces, and P, the distance of the 

 plane of maximum distinctness from the surfaces. If 6 be small, the 

 variations of P with 6 may be neglected, and we have then 



P=-i5_tan i, 

 tan ^ 



or with sufl&cient accuracy, 



P=^-?i. 



<i> 



' In the case of close groups of lines the image of the source is first thrown on a 

 slit, otherwise the lines at S would overlap. 



- The light entering the telescope is a maximum when the thickness of the silver 

 film is such that the intensity of the transmitted light is equal to that of the reflected 

 light. The silvering has another important advantage in diminishing the relative 

 intensity of the hght reflected from the other surface. Indeed, for this purpose it 

 is advisable to make the tilm heavier ; even so thick that the reflected light is twice 

 as bright as the transmitted. This does not atTect the ultimate ratio of intensities 

 of the interfering pencils — for what is lost by transmission on entering the plate G, 

 is made up by reflection on leaving it, the effect being simply to diminish somewhat 

 the whole intensity. Another advantage of the thicker film is that it can be made 

 uniform with far less ditficulty than the thin film. It may be mentioned that with 

 this form of instrument the interference fringes in white light present a purity and 

 gorgeoui-ness of coloration that are surpassed only by the colours of the polariscope. 



' The second plane parallel plate G„ is made of the same thickness as the first, 

 and is required to equalise the optical paths of the two pencils. 



