604 



SCIENCE. 



[N. S. Vol. XII. No. 303. 



The inventioa of the telescope is to me 

 the most beautiful ever made. Familiarity 

 both in making and in using has only in- 

 creased my admiration. With the excep- 

 tion of the microphone of the late Professor 

 Hughes, which enabled one to hear other- 

 wise inaudible sounds, sight is the only sense 

 that we have been able to enormously in- 

 crease in range. The telescope enables one 

 to see distant objects as if they were at, say, 

 one five-thousandth part of their distance, 

 whilst the microscope renders visible ob- 

 jects so small as to be almost incredible. 

 In order to appreciate better what optical 

 aid does for the sense of sight, we can im- 

 agine the size of an eye, and therefore of a 

 man, capable of seeing in a natural way 

 what the ordinary eye sees by the aid of a 

 large telescope, and, on the other hand, the 

 size of a man and his eye that could see 

 plainly small objects as we see them under 

 a powerful microscope. The man in the 

 first case would be several miles in height, 

 and in the latter he would not exceed a 

 very small fraction of an inch in height. 



Photography also comes in as a further 

 aid to the telescope, as it may possibly be 

 to the microscope. For a certain amount 

 of light is necessary to produce sensation in 

 the eye. If this light is insufficient nothing 

 is seen ; but owing to the accumulative ef- 

 fect of light on the photographic plate, 

 photographs can be taken of objects other- 

 wise invisible, as I pointed out years ago ; 

 for in photographs I took in 1883 stars 

 were shown on photographic plates that I 

 could not see in the telescope. All photo- 

 graphs, when closely examined, are made 

 up of a certain number of little dots, as it 

 were, in the nature of stippling, and it is a 

 very interesting point to consider the rela- 

 tion of the size and separation of these dots 

 that form the image, and the rods and 

 cones of the reckoner which determines the 

 power of the eye. 



Many years ago I tried to determine this 



question. I first took a photograph of the 

 moon with a telescope of very short focus 

 (as near as I could get it to the focus of the 

 eye itself, which is about half an inch). 

 The resulting photograph measured one 

 two-hundredth of an inch in diameter, 

 and when examined again with a micro- 

 scope showed a fair amount of detail, in fact, 

 very much as we see the moon with the 

 naked eye ; making a picture of the moon 

 by hand, on such a scale that each separate 

 dot of which it was made correspbnded 

 with each separate sensitive point of the 

 retina employed when viewing the moon 

 without optical aid, I found, on looking at 

 this picture at the proper distance, that it 

 looked exactly like a real moon. In this 

 case the distance of the dots was constant, 

 making them larger or smaller, forming the 

 light or shade of the picture. 



I did not complete these experiments, but 

 as far as I went I thought that there was 

 good reason to believe that we could in this 

 way increase the defining power of the eye. 

 It is a subject well worthy of further con- 

 sideration. 



I know that in this imperfect and neces- 

 sarily brief address I have been obliged to 

 omit the names of many workers, but I 

 cannot conclude without alluding to the 

 part that the Association has played in 

 fostering and aiding astronomy. A glance 

 through the list of money grants shows that 

 the help has been most liberal. In my 

 youth I recollect the great value that we 

 put on the ' British Association Catalogue 

 of Stars ' ; we know the help that was given 

 in its early days to the Kew Observatory ; 

 and the reports of the Association show 

 the great interest that has always been 

 taken in our work. The formation of a 

 separate Department of Astronomy is, I 

 hope, a pledge that this interest will 

 be continued, to the advantage of our 

 science. 



A. A. Common. 



