204 



Prof. C. Pritchard. 



Thirdly. If it be established that the photographic film is perma- 

 nent, we possess in the application of photography the certain and 

 definite means of comparing at pleasure any changes which in the 

 lapse of time (whether longer or shorter) may occur in the lustre of 

 any star or group of stars, without the labour of continually 

 measuring and recording the actual magnitude at any particular 

 epoch. We possess, that is, a register, which can be examined when 

 the emergency arising from any suspicious circumstance occurs. 



II. — On the Relation of the Dimensions of the Photographed Images of 

 a Star to the Time of the Exposure of the Plate. 



It will be observed that so far as the foregoing investigations are 

 concerned, the stars examined and compared are all on the same plate 

 and exposed for the same time ; and it is from such plates that the 

 remarkable relation between the areas of the images and the photo- 

 metric magnitude has been obtained. There is, however a still wider 

 question than this, involving the alterations in the disk-areas caused 

 by the variations of the time of exposure of any particular plate on 

 which they are impressed. I do not propose to enter upon this 

 question to the exhaustive extent which at the present day is required, 

 by the extreme and hitherto unexpected duration of the times of 

 exposure, extending now to hours rather than to minutes or seconds. 

 I will here only explain that the question of the relation of time 

 of exposure to the photographic diameter of a particular star im- 

 pressed on a film was elaborately examined by Bond.* From 

 exposures varying from one second to two minutes, he came to the 

 conclusion that the area of a given star's photographic image 

 impressed on a given plate varied directly as the time of exposure. 

 The plates he used were the ordinary wet collodion plates of 1858. 

 My own investigations, based on exposures even when limited to the 

 same time as Bond's (but on the modern gelatine dry plates), led 

 me to a very different conclusion, viz., that the area varies as the 

 square root of the time of exposure. 



Before, however, publishing the details of my observations, it is 

 desirable to extend the time of exposure to far larger durations, and I 

 abstain at present from further remarks thereon, with the exception 

 that, judging from the remarkable photographs taken by the Brothers 

 Henry at the Paris Observatory, Bond's conclusions appear to me (as 

 at present advised, but subject to further examination) incompatible 

 with observed fact. There are also some still more remarkable and 

 interesting investigations relating indirectly to the same question, 

 made recently at Potsdam by Dr. Lohse ("Astr. JSTachr.," vol. Ill 

 (1885), col. 147), which may require remark, and I hope at an early 



* " Astr. Naehr.," vol. 49 (1858), col. 81. 



