134 Prof. F. L. 0. Wadsworth on the 



same grating as mounted and used by Rowland, we have the 

 following results. 

 In the field S' :— 



(1) For mounting A-B the aberration at the centre of the 

 field is nearly iiOO times asgreatas with the Rowland mounting; 

 at the edges of the 10° field the aberrations (maximum) are 

 about thirty-five hundred times larger. 



(2) For mounting C-D the aberration at the centre of the 

 field (maximum) is about thirty-seven hundred times as great 

 as with Aq (R. mounting). At one edge of the 10° field it 

 is about sixteen hundred times larger ; at the other it is over 

 five thousand times larger. 



(3) With mounting E the aberration at the centre of the 

 field is seven thousand times larger than with the R. mounting. 

 At one edge it is only twenty-five times larger, but at the 

 other it is again about seven thousand times as great. 



(4) With mounting H the aberrations at the centres of 

 the various fields 8^ vary from the same values already 

 given for the A-B type to a maximum value nearly two 

 hundred thousand times as great as that for the Rowland 

 mounting. 



For the fields W, S^^^, and S^^ the comparisons are more 

 favourable to the O.S. types of mounting, both for the centres 

 and for the edges o£ the fields. In the field S'' for example 

 the ratios 



run as follows 



Zi_2 (O.S. mountings) _^ 

 Zo (Rowland mountings) ~~ ^ 



(1) For the A-B type Q'^ (centre) ^ 26*0 



: Q;;^, (edges) - 830-0 



(2) Tor the O-I) type Q" (max.) ^1502-0 



Q';^ ( . ) = 441-0 



Ql, ( ,, ) ^2233-0 



(3) For the E type Q[J (max.) ^3132-0 



Q';,( . ) ^1760-0 



Q'' ( ,, ) 2r3S10-0 



