Diffraction Spectra described by Prof. Wood. 63 



The numbers headed " observation " are measured from 

 Prof. Wood's diagrams ; but owing to the width and unsym- 

 metrical form of some of the bands they are liable to consi- 

 derable uncertainty. It would appear that (with the exception 

 of the third band in diagram (10)) all the positions are pretty 

 well represented by (2). 



As regards the observations when the face of the grating 

 was cemented to glass with cedar-oil, we have in place of (1) 



e(l±sin0')=nV, 



where X' is the wave-length and 0' the angle of incidence in 

 the oil. Now if fi be the refractive index of the oil, 



X'=X/fi, ffln^=flin0.//A, 

 so that 



e(/x + sin 6)=nX, (4) 



if as usual 6 and X are measured in air. 



In the diagrams of Prof. Wood's fig. (2) there are four 

 angles of incidence. The bands are markedly unsymmetrical 

 and the numbers entered in the following table are those 

 corresponding to the sharp edge. The values for n are calcu- 

 lated from (4) on the supposition that //,= 1'5, the lower sign 

 being chosen if the angles on the first side are regarded as 



0. 



A. 



11. 



12° 8' 



541 



590, 469 

 610, 489 

 655, 529 



403 



3-94, 4-96 

 397, 4-95 

 3-99, 4-93 



7° 8' 



3° 53' 



-2° 29' 





positive. The wave-lengths observed correspond pretty well 

 with the passing off of the fourth and fifth spectra on the 

 opposite side to that upon which the light is incident. There 

 seems to be nothing corresponding to the passing off of spectra 

 on the same side. Upon the whole there appears to be con- 

 firmation of the idea that the abnormalities are connected 

 with the passing off of higher spectra, especially if the 

 suggested value of e can be admitted. 



The argument which led me to think that something peculiar 

 was to be looked for when spectra are passing off may be 

 illustrated from the case of plane waves of sound, incident 

 upon a parallel infinitely thin screen in which are cut 

 apertures small in comparison with X. The problem for a 



