Grubb — On the Construction of Telescopic Object- Glasses. 477 



that the two lenses be of such a form as would touch in centre if placed close 

 together. By continuing the bending a point is reached at which the coma is 

 cured ; if we go further we obtain coma outwards. When the proper balance of 

 coma is obtained we have an objective of approximately the form shown in fig. 6.* 

 But, unfortunately, whichever course we adopt, either that of bending forward 

 the crown or bending backward the flint, the correction for spherical aberration 

 is destroyed. Bending forward the crown renders it (the crown) a lens of greater 

 positive spherical aberration, and bending back the flint renders it (the flint) a 

 lens of less negative aberration. In either case the spherical aberration of the 

 whole is rendered strongly positive. 



Again, in order to correct the objective for coincidence of the active chemical 

 rays, it is necessary either to increase the power of the crown, or reduce the 

 power of the flint; and this again introduces more positive spherical aberration 

 into the objective, the result being a lens, corrected for chemical rays, corrected 

 for coma, but under-corrected by a considerable quantity for spherical aberration. 

 In an ordinary visual objective if the spherical aberration be found to be 

 under-corrected, a modification of the form of either of the components should be 

 made in order to correct this, but in this case all four curves are already fixed 

 in order to fulfil the three conditions of focal length, chromatic aberration, and 

 spherical aberration, consequently there is no latitude for fm-ther alteration. The 

 total power of the combination is already fixed in order to fulfil the condition of 

 focal length. The proportion of the power of crown to flint is already a fixed 

 quantity in order that the chemical rays be united. If therefore the total power of 

 the combination, and the proportional power of crown to flint be fixed, the actual 

 powers of crown and flint are also fixed quantities. Lastly, the proportion of 

 power on each surface of crown and flint, i. e. the form of the lens, is either fixed, 

 or variable only in such directions as will not sensibly alter the correction for 

 spherical aberration of the whole combination, that is to say, the correction for 

 coma may be made as before stated, either by bending forward the edge of the 

 crown, or bending back the edge of the flint, or by a combination of both, but in 

 any case the correction for spherical aberration is reduced. 



As there is no possibility of altering the curves of ant/ surface of the objective 

 without destroying the correction for one or other of the foregoing, the only 

 possibility of correcting the spherical aberration is by figuring the surfaces to a 

 form other than that of a sphere, and this is the course I was obliged to adopt 

 when using this particular form of objective. 



* Tte same effect on coma can also be obtained by separating tbe cro-wn and flint, but in most cases the 

 amount of separation required is excessive, and introduces more positive spherical aberration than the other 

 method. 



