CASSEGKA1N KKFLECTOU WITH CORRECTED FIKLD. 61 



seven fold before impact upon this surface. It is this that produces the large 



It might be possible, with these numbers before one, to rearrange the general plan 

 of the surfaces so as to produce a smaller value of <J 3 H, but as explained above, it is not 

 essential to do so in a telescope which is not likely to I* used for exact measures over 

 a field of more than 30 minutes radius. 



We now return to the question of achromatism. We shall first verify that as fur 

 as the normal scheme goes, the achromatism which was secured for the scheme of 

 p. 54, has not been sensibly impaired by the changes since made in the distribution 

 of curvatures between the surfaces. Writing down only the surfaces, we have 



1*5352. 



Surface < > 

 Surf:uvs ( >, ( )., 



,, ' '0' * *2> ' ' 'J 



o,,... <>" 



...0 4 

 ,, 0....0", 



o ...o, 

 IV..O'. 



Normal Srhi-im-s for 



{ +1-000000, 



{+ '339933, +r320i:i:< 



{+ '332299, +1*328811 



{+ '330671, +1-361508 



{+ '141569, + '3325076 



{+ '140298, +3-352749 



{+ '140287, +3-388119 



{+ '140283, +3'457665 



* , +5-088015 



--500000, +r oooooo}, 



-381G9G, + '433886}, 



-'08 1365, +1-634860}, 



-'208547, +2-165477}, 



-'101641, +2'213876}, 



-'002245, +7'074002}, 



-'000281, +4-636412}, 



-'196540, +2'284156}, 



-196540, +2-284156}. 



(37) 



liy comparing this with the schemes (35), p. 58, it will be seen that the rays of 

 different refractive index separate decidedly in the course of their passage through 

 the instrument before they are brought together at their common principal focus. 

 The filial agreement was to be expected as it was within our control, as far as the 

 normal schemes were concerned, but it now remains to be considered whether there 

 is any sensible chromatic difference of aberrations ; this is found by recalculating the 

 aberration coefficients with refractive index 1*5352 in place of 1*5200. The results 

 are as follows : 



n + Sn. 



<J,G = -'00018, (J.G = +''00846, J,G = - '45888. 

 .5,H = +'00844, 4H = + 1-39705, <? 3 H = +351*826. . 



(38) 



