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



During the course of my investigations on these objectives, I worked out 

 another form which gives very admirable results, and without as much or indeed 

 any sensible forcing of the curves out of the spherical form. i 



This objective is shown in fig. 7. In this form the flint is 

 the outermost, and receives the parallel rays ; the two inside 

 curves are, as in the other form, so proportioned that they fig. 



would touch in the centre; and the outside of the crown is nearly piano. This lens 

 when corrected for coincidence of the chemical rays and for coma, is very nearly 

 correct for spherical aberration also, and therefore its preparation is less trouble- 

 some than the form first described. If the inside curves were made to coincide 

 there would be coma, but its direction would be the reverse of that given by the 

 first described form, viz. it would be from, instead of towards, the centre of the field 

 of view. In the vicinity of smoky cities this form might be objected to. Having 

 the flint glass exposed to the influence of the air, it is more likely to be injured 

 than if the crown were outermost. This form corrected for visual rays would make 

 an admirable objective for transit instruments, or in any case where a large field 

 is required. 



The testing and correcting of these photographic objectives is much more 

 troublesome than that of visual objectives. In visual objectives, the judging 

 as to the perfection of the chromatic and spherical aberrations is altogether a 

 matter of experience of the eye of the observer. A very few moments of favour- 

 able observation on a star or small point of light is sufficient to enable the 

 experienced eye to judge of the fault to be corrected in the case of visual 

 objectives, but in the case of the photographic objective no visual observations 

 are of any use whatever, except that the state of the chromatic aberration 

 can be roughly estimated by observing through various coloured glasses, 

 or better still, through a film of ammonio-oxide, or ammonio-nitrate of copper ; 

 but the amount of outstanding colour is so great that no judgment can be 

 formed of the state of the spherical aberration, and for the final tests photographs 

 must be taken at every step of the process. When it is remembered that a clear 

 sky and the utmost perfection of driving mechanism in the most perfect order is 

 necessary in order to obtain these photographs, it can easily be imagined how 

 very great are the difliculties in the preparation of the object-glasses. 



Every step in the final corrections and final adjustments require a photograph 

 to be taken, and consequently some of the simpler adjustments which are generally 

 made within the space of an hour or so in the case of visual objectives, require in 

 the case of photographic objectives a series of photographs to be taken, some- 

 times in consequence of broken weather, extending over several days, or even 

 weeks. 



As I have mentioned before, the final and crucial test for these object-glasses 



