252 Dr. H. Draper on the Photographic Use of 



minute fissures will spread all over them, and the silver seem to 

 lose its adhesion to the glass. This appears to arise from con- 

 tinued exposure to dampness, and may be obviated by covering 

 the concave surface, when not in use, with a plate of flat glass, 

 the edge of the concave being ground flat. The diagonal mirror 

 of my Newtonian is not subject to this difficulty, owing to the 

 free ventilation and greater warmth in its neighbourhood. The 

 lower story of the observatory was contrived to keep the large 

 metal speculum at a uniform temperature, and is excavated out 

 of the solid rock. Being cool, it communicates to objects put 

 in it a tendency to condense moisture from the warmer air that 

 enters. It is obvious that an observatory for a silvered-glass 

 instrument should be altogether above ground, and not cooler 

 than the adjacent air. 



The vapours which arise from fresh paint exercise a prejudicial 

 effect, in depositing upon the surface a somewhat greasy film. 

 After repainting the interior of the buildings, it became neces- 

 sary to expose the large concave several times to the sun, and 

 repolish it to keep it in working order. In the course of three 

 months the trouble sensibly disappeared, the present mirror not 

 having been taken off its air-sac since spring, nor will it require 

 to be moved for a Ions- time. 



o 



The reflecting power of silver films varies considerably with 

 the method of preparation. When deposited from alcoholic solu- 

 tions in the manner recommended by M. Foucault, they have 

 frequently a leaden appearance, while the two processes just 

 described give surfaces of velvety blackness in oblique positions, 

 and certain to show objects in their natural colours. This dulness 

 results probably from the presence of foreign matters incorporated 

 with the silver, as is shown by the fact that when such a film is 

 dissolved off a piece of glass with nitric acid, an insoluble reddish 

 powder is left. 



Tor the purposes of celestial photography, a silvered-glass 

 telescope offers, without doubt, the greatest advantages. Not 

 only is it less difficult to manage than a speculum, but its higher 

 reflecting power materially shortens the time of exposure of the 

 sensitive plate. The superiority of any reflector to an achro- 

 matic is of course too well known to require mention. I have 

 no doubt that they will in future be constructed of much larger 

 size than any speculum yet made, the intrinsic difficulties being- 

 much less. The glass need only weigh one-eighth as much as 

 the metal. They are also more permanent; for if by accident 

 the silver should be injured, it is only a morning's work to dis- 

 solve away the old film and replace it by a new one, which will 

 copy the glass below with so much accuracy that the most refined 

 tests (such as that with an eclipsing screen at the centre of cur- 



