ASTRONOMICAL PHOTOGRAPHY. 169 



enlargement it furnishes the negative which is used to print the paper 

 positives. There are a great number of such along the wall of different 

 phases of the moon enlarged to about eighteen inches. The largest 

 here is nearly one metre or rather over three feet two inches diameter ; 

 and at the time this was made, some years back, we did not possess 

 cameras large enough to produce so large a photograph at one opera- 

 tion, so that it was done on four plates. You will notice that in the 

 successive enlargements there is always a loss of definition, and that 

 more details are to be seen in the original negative, to which recourse 

 must be had for exact measurements. 



I now come to solar photography. There are some early French 

 photographs downstairs ; but solar photography was first systemati- 

 cally followed up at the Kew Observatory, in consequence of a 

 suggestion of the desirability of photographic sun observations having 

 been made by the late eminent astronomer, Sir John Herschel, to the 

 Royal Society. The Kew photo-heliograph is before you. This 

 instrument was in use about 1854 at Kew, but was not systemati- 

 cally worked. It was taken to Spain in 1860, and there used for 

 photographing the solar eclipse. It was afterwards worked from 

 1862 to 1872 systematically in observing the sun's spots. Eleven 

 instruments on the Kew model, but improved, have since been con- 

 structed by Mr. Dallmeyer under my direction, and took part in 

 the observations of the transit of Venus. The more recent photo- 

 heliographs have the object glass four inches in diameter ; this is 

 about three and a half. The image is not allowed, as in the case 

 of lunar photography, to fall directly on the sensitive plate, but 

 passes through a secondary magnifier, and is thus enlarged to four 

 inches. In the common focus of the object glass and the magnifier 

 are cross wires, most conveniently placed at the angle of 45 to a 

 terrestrial meridian passing through the sun's centre, and a negative of 

 the sun like that I show you has the cross wires depicted upon it. 

 They are of great use in the subsequent measurements for obtaining 

 the position angle from the north of any spots that may be depicted. 

 The pictures obtained during ten years are now undergoing measure- 

 ment by this micrometer, which I will describe. It has a divided 

 circle, on which the photograph to be measured is placed and fixed ; 

 with the aid of the microscope it is then adjusted so that the centre of 



