PHOTOGRAPHING THE STARS. 309 



resulting photograph will be entirely wanting in delicacy 

 Xor will a mere alteration of the place of the plate suffice 

 to give precision to the image, for there are so many dif- 

 ferent shades of photographic light that an ordinary ob- 

 jective when focussed for one kind of invisible light will 

 be out of focus for another. 



For photographic purposes we must therefore entirely 

 reject the familiar objective of the observatory, and con- 

 struct a different one. All the reds and yellows may with 

 safety be permitted to run wild, inasmuch as their photo- 

 graphic capacities are insensible. But the true chemical 

 rays, beginning in the blues and the violets and extending 

 far off into the invisible portion of the spectrum, must be 

 carefully gathered into one point. A pair of flint and 

 crown lenses must thus be so wrought that the two ends 

 of the chemical parts of the spectrum shall be practically 

 brought to a common focus, in which, of course, the 

 photographic plate is to be placed. 



We thus obtain an objective which is utterly unsuited 

 for visual purposes, but which will give an exquisitely de- 

 fined photographic image of a star. But now comes one 

 of the practical difficulties of the optician. In forming 

 the visual objective it is easy for him to test the successive 

 approaches to the perfect form of the lenses, but how is he 

 to test the performance of the photographic objective? 

 Here the eye cannot directly appreciate the degree of 

 success which has been obtained. 



At the request of Signer Anguiano, the present writer 

 has tested the large photographic objective constructed by 

 Sir Howard Grubb for the Observatory of Tacubaya be- 

 longing to the Mexican Government. A description of 



