ON THE ASTIGMATISM OF ROWLAND'S CONCAVE GRATINGS. 



By Dr. J. L. SIRES, Groningen. 



In a well-known paper of Mr. J. S. Ames, On Concave Gratings 

 for Optical Purposes a ), the following' passage occurs. „ Owing to 

 „ the astigmatism of the grating, it is not possible to adopt the usual 

 „ method of illuminating part of the slit with the solar image and 

 „part with the spark or arc; and so a different and far better plan 

 „ is adopted. A compound photograph of the two spectra is taken 

 „ in the following manner." 



Yet this new plan, devised and executed by Professor Rowland 

 with his wonted success, is only applicable by means of photography, 

 as the photographs of the different spectra must be taken one after 

 another; and if the precited statement, — which, so far as I see, 

 neither Mr. Ames nor Prof. Rowland, at whose request he wrote, 

 has recalled or modified — were to be accepted in its apparent 

 purport, the beautiful instrument with which Mr. Rowland has 

 endowed the spectroscopist would be unfit for the direct comparison 

 of spectra from different sources by ocular observation, that was 

 always regarded as a precious function of the dioptric spectroscope. 



Fortunately however, though in the literal acceptation of the words 

 it is useless to illuminate part of the slit with one source of light 

 and part with another, it is certainly possible to institute the 

 intended comparison, at least with the first and second spectra, by 

 a slight modification of the common method: the prisms or other 

 equivalent contrivances that are generally used to introduce lateral 

 beams of light, need only be placed not against the slit, in A (Fig. 1), 

 but at a distance q sec v — q cos v from the slit, q sec v from the gra- 

 ting, viz. at a point Q, being the intersection of B A and the tangent 

 in the focus C. 



') Phil. Mag. XXVir, p. 381, 1889; cf. Astr. and Astro-Ph. 1892, p. 39. 



F 1 



Verhand. Kon. Akad. v. Wetensch. (Ie Sectie). Dl. U. 



