1891.] 



on Some Applications of Photography, 



269 



why large telescopes are necessary, is, because it is only by their 

 means that we can get enough light. That may be in some cases a 

 sufficient reason, but that it is inadequate in others will be apparent, 

 if we consider the case of the sun. Here we do not want more light, but 

 rather are anxious to get rid of a light already excessive. The prin- 

 cipal raison dj'etre of large telescopes, is, that without a large aperture 

 definition is bad, however perfect the lenses may be. In accordance 

 with the historical development of the science of optics, the student 

 is told that the lens collects the rays from one point to a focus at 

 another; but when he has made further advance in the science he 

 finds that this is not so. The truth is that we are in the habit of 

 regarding this subject in a distorted manner. The difficulty is not to 

 explain why optical images are imperfect, no matter how good the 

 lens employed, but rather how it is that they manage to be as good 

 as they are. In reality the optical image of even a mathematical 

 point has a considerable extension ; light coming from one point 

 cannot be concentrated into another point by any arrangement. There 

 must be diffusion, and the reason is not hard to see in a general way. 

 Consider what happens at the mathematic£tl focus, where, if anywhere, 

 the light should all be concentrated. At that point all the rays 

 coming from the original radiant point arrive in the same phase. The 

 different paths of the rays are all rendered optically equal, the greater 

 actual distance that some of them have to travel, being compensated 

 for in the case of those which come through the centre by an optical 

 retardation due to the substitution of glass for air ; so that all the 

 rays arrive at the same time.* If we take a point not quite at the 

 mathematical focus but near it, it is obvious that there must be a 

 good deal of light there also. The only reason for any diminution at 

 the second point lies in the discrepancies of phase which now occur ; 

 and these can only enter by degrees. Once grant that the imvige of a 

 mathematical point is a diffused patch of light, and it follows that 



Description of Fig. 8. 



A, B, Electrodes of Wimshurst machine. 



C, D, Terminals of interior coatings of 

 Leyden jars. 



E, F, Balls on insulating supports be- 

 tween which the discharge is 

 taken. 



G, Attracted disc of electrometei*. 



H, Knife edge. 



I, Scale pan. 



J, Stops limiting movement of beam. 



K, Sparking balls in connection with 

 exterior coatings of jars. [These 

 exterior coatings are to be joined 

 by an imperfect conductor, such 

 as a table.] 



L, Lantern condenser. 



M, Soap film. 



N, Photographic camera. 



O, Daniell cell. 



Q, Electromagnets. 



P, Key. 

 R, Balls. 



* On this principle we may readily calculate the focal lengths of lenses with- 

 out use of the law of sines. See ' Phil. Mag.,' Dec. 1879. 



Vol. XIII. (No. 85.) T 



