1888.] Silver on Glass Mirrors of different Focal Lengths. 173 



Science, inasmuch as these small discrepancies are well within the 

 limits of the errors of observation. 



The conclusions to be drawn from these results thus obtained are : 

 1. The very slight deterioration of the mirrors after nine months' 

 constant use and exposure. 2. The very considerable amount of 

 light reflected by these mirrors when compared with that transmitted 

 by the Grubb object-glass, amounting in fact to this, that a mirror of 

 15 inches aperture affords an image of a star as brilliant as that 

 formed by an object-glass (of the particular quality presented) of 

 13*35 inches aperture. 3. A slightly increased, but only a slightly 

 increased, luminosity of image is caused by the adoption of the focal 

 length of 80 inches instead of 120. The result, referred to above 

 in 2, is in conformity with the remark made by Dr. Robinson, in 

 'Phil. Trans.,' vol. 151, to the effect that in respect of the luminosity 

 of the image, Newtonian reflecting telescopes of attainable aperture 

 would probably surpass refractors of attainable dimensions, on 

 account of the increasing absorption of light, by reason of thickness, 

 unless indeed the translucency of glass can be sensibly improved. 



It is to be noticed that with an exposure of half an hour in the 

 -|§-inch mirror, the existence of nebulosity in the neighbourhood of 

 Maia is distinctly traceable on the photographic plate. With the 

 exposure of an hour it is observable in form. No trace of the fainter 

 nebulosity near Merope has been impressed. 



III. The Angular Extent of Apparently Undistorted Field, and the 

 Amount of Distortion ivhere it Exists. 



The determination of these elements is of the utmost importance 

 in the formation of charts of the heavens by the aid of photography, 

 inasmuch as on the superficial extent of the reliability of the photo- 

 graphic field depend the timfi, labour, and cost of charting the 

 heavens. A general idea of this extent of reliable field may be 

 gathered from the quality of the stellar images impressed at different 

 distances from the centre of the plate. Thus in the case of the 

 ^-inch mirror at about forty minutes from the centre of the plate 

 the star images cease to be sufficiently circular, although for a short 

 distance beyond, distances between these stellar disks may still be 

 measured, though not possessing the utmost reliability. In the case 

 of the Y^-inch mirror, this angular extent of measurable field extends 

 beyond fifty-one minutes from the centre. This very perceptible supe- 

 riority of the y^-inch mirror arises, partly, no doubt, from its longer 

 focus, and it may also be influenced by the effects of the intervention 

 of the plate holder ; but be the causes what they may, the superiority 

 longer focus is unquestionable in respect of extent of field. 



But an equally important element remains to be investigated, 

 namely, the amount of distortion which exists at different distances 



