6 THE PRESIDENTIAL ADDRESS 



structure. We may compare the picture on the retina to a design 

 embroidered in woolwork, which also has a structure. Clearly such a 

 design cannot embody details which are smaller than the mesh of the 

 canvas which is to carry the coloured stitches. The only way to get 

 in more detail is to make the design, or rather such diminished part 

 of it as the canvas can accommodate, on a larger scale. Similarly 

 with the picture on the retina. The individual rods and cones 

 correspond with th3 individual meshes of the canvas. If we want 

 more detail of an object we must make the picture on the retina 

 larger, v^^ith the necessary sacrifice of the field of view. If the object 

 is distant we want for this a lens of longer focus instead of the eye 

 lens. We cannot take the eye lens away, but, what amounts to nearly 

 the same thing, we can neutralise it by a concave lens of equal power 

 put right up to it, called the eyepiece. Then we are free to use a long 

 focus lens called the telescopic objective to make a larger picture on 

 the retina. It must of course be put at the proper distance out to 

 make a distinct picture. This is a special case of the Galilean 

 telescope which lends itself to simple description. It is of no use 

 to make the picture larger if we lose definition in the process. The 

 enlarged image must remain sharp enough to take advantage of the 

 fine structure of the retinal screen that is to receive it. It will not 

 be sharp enough unless we make the lens of greater diameter than 

 the eye. Another reason for using a large lens is to avoid a loss of 

 brightness. 



It seems paradoxical that the image of a star should be smaller 

 the larger the telescope. Nevertheless it is a necessary result of 

 the wave character of light. We cannot see the true nature of, 

 for example, a double star unless the two images are small enough 

 not to overlap and far enough apart to fall on separated elements of 

 the observer's retina. 



When the problem is to examine small objects we look at them as 

 close as we can : here the short-sighted observer has an advantage. 

 By adding a lens in front of the eye lens to increase its power we can 

 produce a kind of artificial short sight and get closer than we could 

 otherwise, so that the picture on the retina is bigger. This is a 

 simple microscope and we can use it to examine the image produced 

 by an objective lens ; if this image is larger than the object under 

 examination we call the whole arrangement a compound microscope. 



Given perfect construction there is no limit in theory to what a 

 telescope can do in revealing distant worlds. It is only a question 

 of making it large enough. On the other hand, there is a very 

 definite limit to what the microscope used with, say, ordinary daylight 

 can do. It is not that there is any difficulty in making it magnify 

 as much as we like. This can be done, e.g., by making the tube of 

 the microscope longer. The trouble is that beyond a certain point 



