162 Mr. A, A. Common [May 30, 



the wonderful effect of increasing the optical power is perfectly correct 

 as far as it is concerned. We will deal with a part only of the moon, 

 representing, as I have just said, about one-tenth of its diameter, or 

 one-hundredth of its visible surface. Two such portions of the moon 

 are marked, as you see, on the diagram. I have selected these por- 

 tions as I am able to show you them just as taken on a large scale by 

 photography so that you can make the comparison in the most certain 

 manner ; but let us first analyse our diagrammatic moon — let us 

 magnify it about ten times, and see what it looks like. 



I now show you a picture of this part of the diagram, inclosing 

 the portions I wish to speak about, magnified ten times, so that you 

 can see that about twenty-eight of our points, and by supposition 

 twenty-eight of our particles of silver on the photographic plate, 

 make up the picture. You will see that these dots vary in size ; the 

 difference is due to the amount of light falling within what we may 

 call the sphere of action of each point, and should represent it exactly. 

 The result can hardly be called a picture, as it conveys no impression 

 of continuity of form to the mind. We have got down to the structure 

 or separate parts, and to the limit of the powers of the eye and the 

 photographic plate, of course on the assumption we have made as to 

 the size of the points in the one case and the particles of silver in the 

 other. I will now show you the same parts of the moon as rej)re- 

 sented by the circles on our diagram exactly as delineated by photo- 

 graphy. You now see a beautiful picture giving mountains, valleys, 

 craters, peaks, and plains, and all that makes up a picture of lunar 

 scenery. We have thus seen how the power of the eye is increased 

 by the enlargement of the picture on the retina by the telescope, and 

 also how, by increasing the size of the photograph, we also get more 

 and more detail in the picture. 



We know we cannot alter the number of those separate points on 

 the retina which determine the limit of our powers of vision in one 

 direction, but we may be able to increase enormously the number of 

 particles of silver in our photographic picture by processes that will 

 give finer deposits, and so, in conjunction with more perfect and 

 larger photographic lenses, we may reasonably look for a great 

 improvement in our sense of vision — it may be even beyond that 

 given by the telescope alone ; although it always will be something 

 in favour of the telescope that the magnification obtained in the eye 

 is about fifteen times greater than that obtained by photography 

 when the image on the retina is pitted against the photograph of the 

 same size, unless we use a lens to magnify the photograj)h of the 

 same focal length as the eye, in which case it is equal. But we may 

 go much further in our magnification of the ^photographic image. In 

 other ways there is great promise when we consider the difference 

 in the action of the eye and the chemical action in the sensitive film 

 under the action of light. As I pointed out in the discourse I gave 

 about four years ago in this theatre, the eye cannot perceive objects 

 that are not sufficiently illuminated, though this same amount of 



