UY A. L MoArLAT, B.Sc.. B.A., Ph.D. 99 



will not be exactly that deduced for smaller angles owing to 

 the influence of obliquity, but the error introduced will not 

 be large. 



The spectra will appear at positions given by fin e 



in \ 



(section 6), or if :^ is the number of lines per cm. sin 9~ 

 and their intensities will be as the ordinates of the 



10 

 visibility curve for corresponding values of 6. 



The resolving power of a grating is defined as the re- 

 ciprocal of the fraction of a wave length that separates two 

 spectral lines which the grating can just exhibit as distinct, 

 that is, if the lines have wave lengths X hiuI ,\ -,- d A the re- 

 solving power is ^_- Experience indicates that if the maxi- 

 mum cf one line falls on the first minimum of a line adjacent 

 to it, the two lines can just be recognised as distinct This, 

 therefore, is taken as the criterion of resolution. Let two 

 lines that a grating just resolves have wave lengths \ "lul 



X + (1X. Then as .sin 0= '" ^' ^ (section 6) c<,.s ^ d e = '" ^' ''^ 



10 



ainl (1 6, the angle by which they are separated is 

 ni N (1 X /lO cos 6. Now the angle between the maximum of 

 a line and its first minimum is (16= (section 8). 



10 C...S 



Then, as the two lines under consideration arc just resolved, 

 these two values of d fl must be equal, and , — -^ 



I', i-os o \'j ci)8 a 



or - luN, i.e., the resolving power is the pi'oduct of the 

 il X 



order of the spectrum, and the total number of lines in the 

 grating. 



The foregoing discussion applies equally well to reflection 

 grating?, and with slight modifications to the case of obliqut 

 illumination. 



References. 



Baly, Spectroscopy, Chap. VI. 



Houston, Treatise on Light, p. 171 to p. 180. 



Questions. 



In a certain transmission grating the transparent spaces 

 are the same v/Jdth as the opaque spaces. Where on the 

 visibility curve do the second and third order spoctra lie? 



