1888.] Silver on Glass Mirrors of different Focal Lengths. 171 



impressed by these mirrors is the tendency of those formed from 

 bright stars, to spread themselves over a larger portion of the film in 

 the short focus mirror, and consequently to increase the difficulty of 

 bisection. In the smaller stars, this peculiarity is not so apparent. 

 I am here, contrary to my wont, unable to appeal to numerical data, 

 so essentially necessary in discussions of this description, and where 

 mere estimates and impressions are apt to mislead the judgment. 

 The impossibility of procuring photographs of the same star from the 

 two mirrors under exactly similar circumstances, and therefore of 

 eliminating the relative amount of sensitiveness of the plates em- 

 ployed, the character of the night, and many other circumstances 

 which occur in stellar photography, render the test of numbers im- 

 practicable. I state here the experience gained from the examination 

 of many photographs ; and in immediate connexion with this point 

 of experience, I may mention that the conclusion has been forced 

 upon me, that the images formed from a de la Rue metallic mirror 

 are harder and less extended than those formed from equal exposures 

 on a silver on glass mirror. If I were to hazard an opinion, expressed 

 not without reserve, I should say that the difference between the 

 action of a metallic mirror and a silver on glass mirror, may not 

 unfitly be compared to the difference between the action of a metallic 

 mirror, and the action of such photographic object-glasses as have 

 come under my own observation. 



II. The Relative Luminosity of the Images of Stars, formed by the Two 



Mirrors. 



The mirrors were originally silvered by Mr. Browning, about 

 March 19th, 1887. They were in constant use until January 26th, 

 1888, and on that date the -j^-inch mirror was examined as to its 

 light-reflecting capacity. The secondary plane reflector was silver 

 on glass. The method of determination was the comparison of the 

 places of extinction in the wedge photometer of three stars viewed 

 respectively in the -^Vineh mirror, in the 12^-inch Grubb refractor, 

 and in the 4-inch finder attached to the latter. Each star was 

 extinguished five times in each observation. The method of compu- 

 tation adopted in the light comparison was that explained in the 

 ' Memoirs of the Royal Astronomical Society,' vol. 47. 



The results are as follows : — 



j Light reflected by T Winch mirror _ 



Light transmitted by 1 2^ -inch refractor 

 jj Light reflected by T V 5 o -inch mirror _ 



Light transmitted by 4- inch refractor 



This mirror was subsequently re-silvered at the Observatory by 

 Mr. Jenkins, the film deposited being excellent, February 6th, 1888, 



