1888.] Silver on Glass Mirrors of different Focal Lengths. 169 



mirrors ; 2. The difficulty of adjusting the camera or plate holder 

 perpendicularly to the axis of the mirror, on a temporary mounting, 

 and distant from the workshop of the optician ; 3. An abnormal sky 

 which has continually perplexed astronomers during many months. 



It must not be overlooked, that even the considerable precision, 

 necessary or desirable in the clock motion of a telescope used for 

 micrometrical measures, is comparatively useless for astronomical 

 photography ; for in this latter case the momentary swerving of the 

 telescope through even a second or two of arc, may be fatal to the 

 circular form of the star images impressed on the plate ; and, more- 

 over, it is necessary to maintain this accuracy of steady motion, not 

 merely for a very few minutes at a time, but occasionally for half an 

 hour, or a full hour, or even more. It is true that resort may be had, 

 and in fact must always be had, to the old method of supplementing 

 the driving machine by the occasional assistance of eye and hand ; 

 but unless that machinery is approximately perfect, the strain upon 

 the observer's attention becomes practically insupportable. This 

 perfect steadiness of motion is also necessary from another point of 

 view, because in its absence, it will not be easy to distinguish 

 between the effects of unsteady motion and any optical defect of the 

 mirror. Happily these difficulties have been at length overcome ; and 

 in the month of January last, by the aid of an improved screw, worked 

 on a new engine by Sir H. Grubb, and a subsidiary electrical control 

 connecting the driving apparatus with a seconds pendulum, I had 

 the pleasant satisfaction of hearing from Mr. Jenkins, the assistant 

 chiefly engaged in the present operation, that he now felt no severe 

 strain or stress of attention in watching and occasionally aiding the 

 motion, during the space of an hour or more on the rare occasions 

 when the variability of the sky permitted such long exposures. 

 I am not here speaking of my own experience alone, but I have 

 reason to know that the same troubles have been shared to a greater 

 or less extent by all the few eminent observers who are in this 

 country employed in a similar pursuit. A modification of the 

 ingenious contrivance by which the desired effects have been pro- 

 duced has been recently exhibited by Sir H. Grubb at the Royal 

 Astronomical Society and at the Society of Arts in London. 



The mirrors referred to above, were mounted in succession on the 

 tube of the large equatorial in the Oxford University Observatory, 

 and they proved to be of that excellent optical quality which might 

 be expected in Mr. With's best performance. 



The points to which I chiefly directed my attention in the examina- 

 tion of these mirrors were as follows : — 



I. The general character of the stellar images impressed by the 

 two mirrors, absolute and comparative. 



