MR. BARLOW ON THE NEGATIVE ACHROMATIC LENS. 207 



it is not impossible the negative secondary spectrum of this lens may, by careful ex- 

 periment, be so proportioned as in part to counteract the positive secondary spectrum 

 of the object-glass so as to render the image more nearly aplanatic ; some experiments, 

 at all events, directed to this inquiry are very desirable. 



I have already, in my letter to Mr. Dollond, given the formulae for computing the 

 proper curves according to any distance between the focus and the lengthening lens, 

 and for magnifying the image in any required proportion ; but unfortunately the cal- 

 culation is very laborious, and difficult to be rendered general, or tabulated for ge- 

 neral practice. I would therefore recommend opticians to use the same curves as 

 are commonly adopted for short telescopes of six, eight, or ten inches, making those 

 of the plate or crown concave instead of convex, and those of the flint convex instead 

 of concave, turning the plate towards the object-glass and the flint towards the eye- 

 piece, which will in general bring out a close approximation for spherical aberration, 

 and the colour will be sure to be corrected. Starting from this point, practical skill 

 will readily supply the means of making corrections, if any such should be found 

 necessary after all has been done that can be done by changing the position of the 

 lens as regards its distance from the eyepiece. I hope these additional directions for 

 constructing and applying the lengthening lens will not be thought superfluous, nor 

 undeserving the attention of practical opticians. 



