PLEASURES OF THE TELESCOPE. 221 



protected from the weather by means of a wooden hood or a rub- 

 ber sheet, while the tube of the telescope may be kept indoors, 

 being carried out and placed on its bearing only when observa- 

 tions are to be made. With such a mounting you can laugh at 

 the observatories with their cumbersome domes, for the best of 

 all observatories is the open air. But if you dislike the labor of 

 carrying and adjusting the tube every time it is used, and are 

 both fond of and able to procure luxuries, then, after all, perhaps, 

 you had better have the observatory, dome, draughts and all. 



The next best thing in the way of a mounting is a portable 

 tripod stand. This may be furnished either with an equatorial 

 bearing for the telescope, or an altazimuth arrangement which 

 permits both up-and-down and horizontal motions. The latter is 

 cheaper than the equatorial and proportionately inferior in use- 

 fulness and convenience. The essential principle of the equatorial 

 bearing is motion about two axes placed at right angles to one 

 another. When the polar axis is in the meridian, and inclined at 

 an angle equal to the latitude of the place, the telescope can be 

 moved about the two axes in such a way as to point to any quar- 

 ter of the sky, and the motion of a star, arising from the earth's 

 rotation, can be followed hour after hour without disturbing the 

 instrument. When thus mounted, the telescope may be driven by 

 clockwork, or by hand with the aid of a screw geared to a handle 

 carrying a universal joint. 



And now for testing the telescope. It has already been re- 

 marked that the excellence of a telescope depends upon the per- 

 fection of the image formed at the focus of the objective. In 

 what follows I have only a refractor in mind, although the same 

 principle would apply to a reflector. With a little practice any- 

 body who has a correct eye can form a fair judgment of the ex- 

 cellence of a telescopic image. Suppose we have our telescope 

 steadily mounted out of doors (if you value your peace of mind 

 you will not try to use a telescope pointed out of a window, espe- 

 cially in winter), and suppose we begin our observations with the 

 pole star, employing a magnifying power of sixty or seventy to 

 the inch. Our first object is to see if the optician has given us a 

 good glass. If the air is not reasonably steady we had better 

 postpone our experiment to another night, because we shall find 

 that the star as seen in the telescope flickers and " boils/' and be- 

 haves in so extraordinary a fashion that there is no more defini- 

 tion in the image than there is steadiness in a bluebottle buzzing 

 on a window pane. But if the night is a fine one the star image 

 will be quiescent, and then we may note the following particulars : 

 The real image is a minute bright disk, about one second of arc 

 in diameter if we are using a four-and-a-half or five-inch tele- 

 scope, and surrounded by one very thin ring of light, and the 



