1886.] Mr. A. A. Common on Photography, &c. 3G7 



WEEKLY EVENING MEETING, 



Friday, February 26, 1886. 



William Huggins, Esq. D.C.L. LL.D. F.R.S. Vice-President, 

 in the Chair. 



A. A. Common, Esq. F.R.S. M.B.l. 



Photography as an Aid to Astronomy. 



In many kinds of astronomical work the old method of direct observa- 

 tion seems likely to be superseded by the use of jDhotography ; and 

 the astronomer of the near future, instead of examining with eye 

 and telescope the various objects of the heavens, will prefer to deal 

 with the automatic records they leave on the sensitive plate. In some 

 work this state of things already exists, and its extension to all kinds 

 seems but a matter of time. It appears strange that any indirect 

 way of seeing an object can be better than the direct way, but in 

 some cases we shall certainly find it to be so. Between the construc- 

 tion of the eye and of the apparatus of the photographer there are 

 many points of great similarity. Both have optical means of produc- 

 ing an image, a camera, or dark chamber to keep out other light than 

 that going to form this image ; and both have a similar screen on 

 which this image is received. It is when we come to consider the 

 action of the screen in dealing with the image that the distinctive 

 diiference becomes apparent. In the eye the retina receives this 

 image, and in some occult way the impressions are carried to the 

 brain. With the sensitive plate the varying amount of light that 

 makes up the images produces corresponding changes in the chemical 

 nature of the film which allows the reproduction of this image in a 

 visible form afterwards. 



As I understand the action of the eye, the retina acts only as a 

 transmitter of the sensations produced on its delicate structure by the 

 image ; and the brain records these sensations in a more or less per- 

 fect manner according to its capacity to deal with all those sensations 

 that are transmitted ; hence the power of the eye is limited by the 

 power of the brain to record, and this is evidently in many cases less 

 than that of the eye to perceive, whilst the power of the eye itself is 

 limited in more than one important point as regards our subject, the 

 retina becoming insensible to an image however perfect if insufficiently 

 illuminated. With the sensitive plate the measure of the eflective 

 light of an image is not, as in the eye, the amount going to form the 

 image, but the total amount that can be accumulated by a sufficiently 

 long exj)osure. Hence, we have the remarkable fact that I have else- 

 where before mentioned, that a certain object such as a star or nebula, 



Vol. XI, (No. 80.) 2 b 



