20 THE POSSIBILITIES OF THE TELESCOPE. 



useless. To give some idea of the nicety of manipulation 

 required, I may state that a speculum is ground and finished 

 up to the finishing touch of fine grinding, as a portion of the 

 surface of a perfect sphere. But this figure would not do at 

 all for the telescope. The marginal rays would be reflected 

 to a different, that is, a shorter focus than the central ones. 

 The radius of curvature needs to increase as it approaches 

 the margin. The figure needs to be parabolic, answering 

 nearly to the end of a long ellipse. But so little does this 

 parabolic figure differ from the spherical, that it is all done 

 in the polishing. And this is especially where the highest 

 still of the manipulator is called for. An extra rub with 

 the palm of the hand may be sufficient to distort its figure 



Then another trouble comes in, increasing enormously with 

 the size. We cannot get size without an immense increase 

 of weight, and then comes the difficulty of supporting in their 

 tubes these heavy masses without flexure or straining, the 

 least degree of which is fatal to definition. 



But now, supposing all these difficulties successfully over- 

 come, there remains yet another obstacle, which is utterly 

 beyond man's power or skill to contend with. We live at 

 the bottom of an ocean of air ; and we could not exist even 

 if we could surmount it, any more than a fish could live out 

 of Tflater. And this ocean is in a state of ceaseless agitation, 

 more or less. Through this medium must pass the rays 

 which go to form the telescopic image. Some idea of the 

 effect of this may be formed by trying to read the inscrip- 

 tion on a coin lying at the bottom of a clear pool that is 

 agitated by the breeze. I have viewed the planet Saturn in 

 the telescope, when, from this cause, it presented the appear- 

 ance of a fire-ball ; huge tongues of flame apparently blazing 

 all round it, flickering and dancing; precisely like those 

 kerosene fire-balls which the boys throw up into the air with 

 such magnificent effect on Queen's birth-nights. It was, to 

 all appearance, a ivorld in a blaze. It is not then to be sup- 

 posed that we can, whenever we please, obtain a clear and 

 distinct view of any celestial object we may wish to inspect. 

 Favourable occasions for this are few and far between. Some- 

 times a lull in the atmospheric waves permits of a momentary 

 glimpse of the object clearly and sharply defined ; just as, in 

 the case of the coin in the pool, a brief lull in the breeze 

 allows you to begin to read the inscription ; but before you 

 get half-way through another puff comes up and blurrs it all 

 over. 



Now this difficulty increases with the size of the telescope, 

 inasmuch as that the larger diameter takes in a greater 

 number of these disturbing atmospheric waves ; for which 

 reason the observer has often temporarily to reduce the 

 effective aperture, that is, the power of his telescope. 



