Grubb — On the Construction of Telescopic Object- Glasses. 479 



is that of its performance on celestial objects. Many trials and experiments were 

 made to get satisfactory artificial stars for testing these glasses, but although these 

 are amply sufficient and perfectly satisfactory for testing visual objectives, they 

 have not been found of so. much use in the testing of photographic objectives. 

 They were used largely in the earlier process, and in case of bad weather were a 

 considerable help in forwarding the work more quickly than could have been done 

 were it necessary always to wait for fine nights. But for final tests nothing but 

 the natural star was perfectly satisfactory. During the testing of seven of these 

 objectives a considerable amount of experience in celestial photography has been 

 obtained, and I take occasion to remark that my experience does not tally exactly 

 with that of Dr. Gill respecting the influence of atmospheric disturbance on the 

 photographed images of the stars. He has stated that, with an exposure of over 

 five minutes, as perfect, or very nearly as perfect, star-plates can be obtained on 

 nights when the atmosphere is highly disturbed as on those when the air is 

 perfectly steady. My experience goes to corroborate this so far as the large stars 

 are concerned, but not as respects the small stars, and I think this is easily 

 explained. Bright stars with long exposures imprint images on the photographic 

 plate of considerable diameter. It is not necessary to discuss whether this increase 

 of diameter is due to some form of halation, or chemical action ; the fact remains 

 that the longer the exposure the larger the image printed on the plate by any star. 

 Consequently it is easy to understand that atmospheric disturbance which causes, 

 as all observers know, a flickering and wavering, and general unsteadiness of the 

 image, will have little or no efPect on the photographic image in the brighter stars, 

 because the amount of that wavering will always be far less than, and well con- 

 tained within, the area of the surface which these brighter stars occupy on the 

 plate ; but, in the case of small stars, and more particularly with very small stars, 

 it is impossible to conceive that a point of light which would under favourable 

 circumstances imprint itself as a speck of not more than 2-io^th to perhaps 4-io^th of 

 an inch in diameter would produce equally perfect impressions on the plate if, 

 during exposure, it wavered about over an area considerably more than its own 

 diameter, and this is, as all observers know, the case on nights of bad definition. 

 I was certainly surprised to find the very excellent images that were obtained of 

 the larger and even moderate-sized stars on nights of exceedingly bad definition, 

 but my experience shows that the images of smaller stars suffer very decidedly 

 from the effect of atmospheric disturbance. 



It may be interesting to record here that trials of a form of objective suitable 

 for use either for a visual or a photographic purpose, proposed by Sir George G. 

 Stokes have proved satisfactory. By separating the lenses of an ordinary visual 

 objective, such as fig. 3, a point will be reached when the necessary correction for 

 photographic rays will be obtained, but the spherical aberrations will then be 



TKANS. EOT. BDB. SOC, N.S. VOL. IV., PAET Vni. 3 X 



