164 SECTION PHYSICS. 



ASTRONOMICAL PHOTOGRAPHY. 



Mr. DE LA RUE : In the Loan Exhibition there are, as you know 

 several astronomical photographs, and some apparatus with which 

 these photographs were taken. There is one telescope which is 

 absent necessarily on account of its enormous size ; but I have a 

 small model of it, the Melbourne telescope, to which I shall presently 

 allude. In speaking of astronomical photography, I wish my audience 

 to understand that it is not merely the pictorial representation of 

 celestial objects that astronomical photography concerns itself with 

 mainly ; it is the production of records which can afterwards be 

 measured, and which afford data for astronomical investigations. 

 I had better, in the first instance, just state what happens. Suppose, 

 for example, we have a lens or a mirror directed on to a celestial 

 object and that the image is received on a sensitive plate. I will 

 imagine we have a fixed star in focus whose image is a very small 

 point indeed, scarcely to be distinguished from specks in the col- 

 lodion. But the lens or mirror not being provided with any movement 

 to follow the star, it would happen that in consequence of the star's 

 apparent path in the heavens, if the atmosphere were perfectly quiet, 

 we should have a straight line impressed on the plate instead of a 

 spot, the line being longer or shorter in proportion to the duration of 

 the exposure ; but what does really happen is that we get an irregular 

 wavy line on account of atmospheric disturbances. Now I will sup- 

 pose we use a telescope mounted equatorially, that is to say, on an 

 axis parallel with the earth's axis, and that we drive it by means of 

 clockwork machinery in order to follow the apparent path of the star 

 perfectly. Then the star would, if undisturbed, stand still in the 

 centre of the field, if it weue placed in the centre to start with, but 

 instead of its being depicted as a single spot, we get a series of dots, 

 in consequence of the agitation of the image arising from the cur- 

 rents of air differently heated and differently refracting, if it were a 

 double star we should get a conglomeration of each of the two pictures, 

 for the same reason. The eye looking at a star sees it sometimes 

 steady and well defined, at others blurred and moving about in diffe- 

 rent positions of the field, but the mind selects the best images, and in 



