DR. R. A. SAMPSON ON A 



Iiit.-rpivtiiig these, as on p. 59, we conclude that for 



fi = 1-5352. 



of Least Circle of Alwrration, -0"'007. 



Comatic radius . 

 Focal radiiiH 

 Distortion 

 DisplaceiiK-n 



Radius of curvature of field, 5"423. 



Tlie effect of the distortion at j8 = '01 will be to draw out the image into a small 

 spectrum of length 7"' 13 G""75 = 0""38. The radius of curvature of the field is 

 decidedly changed ; but the effect of the change as shown in the corresponding 

 displacement of the image-circle is not considerable. 



AXIS 6 



Fig. 2. Whole instrument. Scale 1 : 30. 



It will be remarked that all these numbers run in the sense of ^increasing the 

 uU-rrations ; as there is no minimum property about the original index 1'52, we 

 conclude that the aberrations for smaller indices would be proportionately diminished, 

 and we see that it would have been better to have secured exact agreement for the 

 larger index in place of the smaller one. In estimating the effect we may, for 

 instance, take the following values, which are the indices for CHANCE'S hard crown 

 glass: 



0, D, F, G, 



Indt-x 



T5150, T5175, 1-5235, T5284 ; 



that to say with such a glass two-thirds of the excesses shown in the table above, 

 "Hulls of p. 60, would cover all chromatic differences. There appears to 

 HlBg , any ,,f them that calls for a revision of the calculations 



<", to the question of the actual sizes and places of the mirrors and 



