

ASTRONOMICAL PHOTOGRAPHY. 165 



all astronomical drawings the disturbances are eliminated ; the 

 memory discards those, and the hand only draws the appearances at 

 moments of finest definition. A photographic plate, however, retains 

 all the impressions ; it is a retina on which all the disturbances are 

 permanently recorded, and consequently no photograph yet obtained 

 in any way conveys the sharpness of outline of the finest definitions 

 of the telescope, it is always more or less blurred. Nevertheless very 

 valuable results, as you will presently see, are obtainable. For 

 example, if we want to ascertain the position angle and the angular dis- 

 tance of two double stars, the disturbances do not prevent our finding 

 accurately the centres, and we can get, by means of a micrometer, these 

 data just the same as though there were no blur. 



The size of the focal image of any celestial object, other than a fixed 

 star, which is always a mere point, I need scarcely tell you, depends on 

 the length of the telescope ; in my own telescope the focal length is 

 ten feet, and the image of the moon is about an inch ; I say " about 

 an inch," because it varies in consequence of the nearer approach of 

 the moon to the earth at one time than at another. I have here a 

 group of original negatives of the moon, showing the sizes they are 

 obtained by my telescope when she is in different positions in her 

 orbit. The time of the exposure of these photographs is marked on 

 them, it varies generally from about one second or less than a 

 second, for a full moon, to about eight or ten seconds when at the 

 crescent. The duration of exposure to produce an image depends, 

 with equally sensitive chemicals, mainly on the relation of the aperture 

 to focal length, and hence it is very desirable to get as large an aper- 

 ture as possible with respect to the focal length or, in other words, to 

 make the focus of the telescope as short as is consistent with good defi- 

 nition. It is quite possible, by means of clockwork, to follow a star 

 almost perfectly, that is to say, if a telescope is put on a star and 

 adjusted so that its image falls on the cross wires in the field, one may 

 leave the telescope and come back again after an hour, and find the 

 star there still ; so that that mechanical difficulty to obtaining good 

 photographs is quite overcome. The great enemy, however, is always 

 the atmospheric disturbance ; but with regard to the moon there is 

 another difficulty : the equatorial telescope moves simply in a circle 

 parallel with the earth's equator ; but you all know that the moon's 



