370 Mr. A. A. Common [Feb. 26, 



as the use of a polished surface of silver (usually supported by a thin 

 backing of copper) that has been exposed to the action of iodine so as to 

 form a film of sensitive iodide of silver, that may be acted upon by light, 

 and afterwards developed by the vapour of mercury. Although not 

 now used for astronomical work, it may be that owing to the intimate 

 connection between the image formed by the silver compound and the 

 backing that supports it, it will be found to have advantages in cases 

 where delicate measurements are required, that the other processes 

 may not have. 



The collodion and gelatine (or dry plate) processes are so called 

 from the nature of the medium that is used to carry the sensitive 

 salts of silver during exposure to the action of light. The chief 

 difference between them, beyond the greater sensitiveness of the 

 latter, is that with the collodion plate the process of preparation, 

 exposure, and develoj)ment must be part of a continuous operation 

 taken in due order and time, with all the necessary apparatus, 

 and the chemicals in a proper state of efficiency, ready to hand, 

 conditions not easy to attend to in astronomical photography, and 

 when long exposures are to be given, hardly to be fulfilled. 



With the gelatine process the plates can be prepared beforehand 

 under the best conditions, and almost any time may elapse before 

 use. The exposure of the plate may take place for any length of 

 time, and the development made at any suitable time afterwards ; all 

 advantages for astronomical work that are obvious ; and in addition, 

 there is the important advantage of the greatest sensitiveness, on 

 which, more than anything else, success so much depends when it 

 becomes a question of dealing with a small amount of light. Each 

 process has its particular advantages of w^hich the specialist may avail 

 himself; but the gelatine dry plate is far beyond the others for 

 nearly every class of astronomical work. 



With regard to the instruments and methods of work, while the 

 main principles are adhered to, modifications have to be made to 

 suit different classes of work. These will be briefly mentioned in 

 speaking of the photographs. In all cases there must be an image of 

 the object, and this image must fall exactly on the sensitive plate, 

 and be kept there during exposure. 



The image produced by a telescope reflecting or refracting is 

 generally used direct ; indeed, with the exception of the sun, where 

 the large amount of light rendered enlargement advantageous, until 

 quite recently the primary image v/as always used, it being thought 

 that with the small amount of light generally at disposal it was better 

 to get a picture thus and then enlarge it afterwards than to enlarge 

 the first image and so increase the time of exposure and all the 

 trouble that comes from atmospheric and instrumental tremors, and 

 , other causes. 



We shall see how much has been gained by departing from this 

 old plan and using an enlarged imago when we come to examine the 

 photographs of the planets. 



