570 M. A. AINSLIE ON AN ADDITION TO THE OBJECTIVE. 



it would have if it were corrected for spherical aberration ; but 

 in any case the additional lens is small, and of low power, so that 

 the effect of its spherical aberration is very slight, and only in- 

 volves a small movement of the draw-tube. Since the final 

 adjustment of the draw-tube would in any case be performed by 

 actual inspection of the image, it will be easily realised that the 

 effect of the spherical aberration of the additional lens is quite 

 unimportant. 



With regard to the chromatic effect, I have only used simple 

 uncorrected lenses in my experiments. Even with these, the 

 effect on the colour correction of the objective is extremely 

 small, except perhaps in the case of an additional lens of 

 10 diopters. 



The only effect that is at all noticeable is that with the convex 

 lens in use the "compensation' required in the eyepiece, to 

 do away with the chromatic difference of magnification (present 

 in all lenses having a single front lens) is somewhat diminished ; 

 in the case of the concave lens, it is somewhat increased ; but 

 this effect is only seen if specially looked for, and with ordinary 

 Huyghenian eyepieces would not be noticed. 



There is, however, a curious effect to be seen in some cases, 

 with objectives which under normal conditions show a certain 

 blue tint on the margins of black objects ; and many of 

 the finest achromatic lenses of the present day, noticeably 

 Watson's Holoscopic, show this effect, which indeed I am informed 

 betokens a more than usually good spherical correction, and 

 in consequence more than usually good definition. I have a 

 Holos 25 mm. objective, of measured N.A. 0'31, corrected for the 

 250 mm. tube ; this, on black objects, such as the lines on the 

 Abbe test plate, shows the blue tint I have alluded to. When the 

 correction of this lens is altered to the short tube by the intro- 

 duction of a convex lens of 2 diopters (20 in. focus), the blue tint 

 almost disappears, and the total quantity of outstanding colour 

 is greatly diminished, so that the additional lens has a sort of 

 " apochromatising ' effect ; this is probably due to the fact 

 that the additional lens slightly alters the ' k preferred colour," 

 for which the spherical correction of the objective is carried 

 out ; but to my eyes there is little, if any, loss of definition on 

 this account, and the 27 compensating eyepiece can still be used 

 with advantage. 



