282 Mr. A. A. Michelson on the Application of 



As stated in Part I. of this paper, the observations neces- 

 sary to construct the visibility-curves, from which the distri- 

 bution of light in any approximately homogeneous source is 

 to be deduced, may be made with any form of interference 

 apparatus which allows a considerable alteration in the 

 difference of path between the two interfering streams of 

 light. 



The apparatus actually employed for this purpose was de- 

 signed for the comparison of wave-lengths, and while admi- 

 rably adapted for the observation of visibility-curves, it con- 

 tains many parts not necessary for this use. Fig. 1, Woodcut, 

 presents the plan of an arrangement which, while showing- 

 all the essential parts, is much less complicated. Starting 

 from V, a vacuum-tube containing the substance whose radia- 

 tions 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 rest by passing through the 

 slitS*. 



The light from S is rendered nearly parallel by a colli- 

 mating lens, and then falls on a transparent film of silver, on 



tends to lower the visibility ; but in either case the correct values 

 may be obtained by multiplying by a constant factor. 



In the first case, let e be the intensity of the extraneous light, and V 

 the resulting visibility ; then, by definition, 



v , (Ii+<0-(I 2 +e) 1,-1, . f 2e 1,-1, 



v -(ix+<0+(i 2 +<0 ~ ix+M-a*' i t +i 2 - ? ' v - (Ix+I.Xl+r/ 



whence V = (l+r)V. 



In the second case, let fj be the ratio of intensities of the interfering- 

 pencils ; then it can readily be shown that the resulting intensity is 



and hence t 



I=(l+p a )P+2p(. 



he visibility is 



1+,' 

 .i+p-V 



U cos 3 — S sin 



i»), 



whence V= 



P 





14-p 2 

 If the interfering pencils differ by 25 per cent., the factor —^ differs 



from unity by about 4 per cent. ; so that, in most cases, this cause of 

 error may be neglected. 



* 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. 



