312 Sir Howard Grubb [May 25, 



case of observing the moon (say), when it is necessary to use tinted 

 screens to moderate the brilliancy, this superiority of the larger aper- 

 ture is just as evident, but it may be explained in this way : — 



In a lecture by Dr. Common, delivered in this theatre in May 

 1890, he gave a very neat explanation of the fact that a certain 

 amount of magnification of image is required in order to see a certain 

 amount of detail in that image. He showed that the sensation by 

 which the brain is excited is carried from the retina by an enormous 

 number of fine nerves, which are excited by small bodies called tech- 

 nically " rods and cones," and that each of these produces one distinct 

 sensation as relating to the particular part of the image which falls on 

 that particular part of the retina ; the image, therefore, as presented to 

 the brain is a kind of mosaic, and it is evident that the larger the 

 image that falls on the retina the finer will the mosaic be in jDropor- 

 tion to the details of the image, and therefore the better will the 

 details be appreciated. 



A similar explanation may be given of the different character of 

 the image given by large aperture telescopes and small. The image 

 of a star, as given by a telescope's objective, is not exactly a point ; it 

 is, owing to certain physical reasons which it would be impossible to 

 enter into in this lecture, in the form of a small disc of light which, 

 if the object is of a sufficient brilliancy, is surrounded by diffraction 

 rings. The diameter of this spurious disc depends amongst other 

 conditions on the diameter of the object-glass ; the larger the diameter 

 of the object-glass the smaller the diameter of the disc ; in other words, 

 the discs, as seen in large object-glasses, are smaller than those seen in 

 smaller object-glasses ; or putting it in another way, if a certain size 

 of object-glass be found to give a spurious disc of a certain size, 

 reducing the aperture of the object-glass will increase the size of the 

 spurious disc. 



Every object may be considered to be made up of an infinity of 

 points, of every one of which the object-glass gives an image in the 

 form of a little disc. It is evident that the image that is made up of 

 the larger spurious discs will not be as fine or as delicate, or show as 

 much detail as that made up with the smaller discs. The image of 

 such an object as Jupiter or Saturn as seen in the small telescope, 

 compared with that as seen in the large telescope, will be as a drawing 

 made in the first case with a coarse crayon or stump, to that made in 

 the second case with a finely pointed lead pencil ; or we may compare 

 the first to a very coarse mezzotint engraving, while the second may 

 be compared to the very finest work that can possibly be turned out. 



This is the reason that the larger aperture telescope, even when 

 used with powers corresponding only to those which can be effec- 

 tively used with a smaller instrument, show objects with a clearness 

 and distinctness that it is impossible to obtain in the smaller instru- 

 ment, no matter how perfect the workmanship) may be. 



And now we come to the question of the probabilities of our 



