1890.] on Astronomical Telescopes, 161 



I have put a black dot, large or small, according to the intensity of the 

 light falling on that part of the image as determined by looking at a 

 photograph of the moon. You will see by the picture of this moon 

 the effect produced. It represents to those who are at a sufficient 

 distance the moon much as it is really seen in the sky. 



We can now with a lens of the same focal length as the eye obtain 

 a picture of the full moon exactly of the size of the actual picture on 

 the retina, and if we take a proper photographic process we can get 

 particles of silver approximately of the same sizes as the dots we have 

 used in making our diagram of the moon ; the grouping is not exactly 

 the same, but we may take it as precisely so for our purpose. I have 

 not any photographs of the full moon of this size, but I have some here 

 of the moon about five, seven and eight days old, which give a good 

 idea of what I mean by the arrangements of the particles of silver 

 being like our diagram. 



It is now quite apparent that if we can by any means increase the 

 size of the picture of the moon on the retina or make it larger on the 

 photographic plate, we shall be able to employ more of our points 

 in the retina of the eye or of our particles of silver in the photo- 

 graphic film, and so be able to see more clearly just in proportion 

 as we increase the size of the picture in relation to the size of the 

 separate parts that make it. 



Now the telescope enables us to do this for the eye, and a lens of 

 longer focal length will give us a larger photographic picture. 



Let us assume that by means of the telescope we have increased 

 the power of the eye one hundred times. The picture of the moon 

 on the retina would now be one-half inch diameter, and instead of 

 employing 2800 points to determine its shape, and the various mark- 

 ings upon it, we should be employing 28,000,000 of these points ; and 

 similarly with the photograph, by increasing the size of our lens we 

 shall obtain a picture made up of this enormous number of particles 

 of silver. But we can go further in the magnification of the picture 

 on the retina — we can also use a still longer focus photographic lens. 



A power of magnification of one thousand is quite possible under 

 favourable circumstances ; this means that the picture of one two-hun- 

 dredth of an inch would be now of five inches in diameter, so we must 

 deal with only a portion of it. Let us take a circle of one-tenth of 

 this, equalling one-hundredth of our original picture, which in the 

 eye, unaided by the telescope, would have a diameter of one two- 

 thousandth of an inch, or an area of less than one five-millionth of a 

 square inch. This means that with this magnification, we have in- 

 creased the power so enormously that we are now employing for the 

 photographic picture two thousand eight hundred million particles of 

 silver, and in the eye the same degree of increase in the number 

 of points of the retina employed in scrutinising the picture 

 piece by piece as successive portions are brought into the central 

 part. 



Photography enables me to show that the result I have given of 



Vol. XIII. (No. 84.) m 



