114 



up to a certain size, and could not be used at all with Newtonian 

 reflectors. 



Another and much more convenient method consists in 

 measuring the image of the object glass or speculum %vhich a 

 telescope forms in its solar focus. When we place our eye at a 

 little distance from the eye-piece of a telescope we observe in the 

 centre of the eye-lens a very distinct and well defined small disc 

 of light. This is the image of the object glass or the speculum, 

 as the case may be. Now if we can obtain a correct measure of 

 the diameter of this disc, and also of the diameter of the speculum, 

 by dividing the latter by the former, we have at once the num- 

 ber of diameters which the telescope magnifies. 



For the purpose of accurately measuring this small disc of 

 light as seen on the eye-lens, several kinds of " dynameters," or 

 as some prefer to call them, " dynamometers," have been used. 

 One of the latest, and perhaps the best, is that known as " Ber- 

 thon's dynamometer." This consists of two strips of thin metal 

 with straight edges so fastened together as to leave between them 

 a long narrow triangular slit, one of the edges of which is so 

 graduated and numbered that measures may be made with it 

 down to one-thousandth of an inch. 



Wliile it is admitted that this measuring of the small disc is a 

 very simple and direct method of obtaining the magnifying 

 power, there are still several difiiculties attending its application. 

 For example, a very small disc of light is not the easiest thing to 

 measure, besides it is not possible to measure a circular disc on 

 its exact diameter by placing it between two lines which are not 

 quite parallel, and Mr. Berthon has devised a very simple and 

 efficient means by which these difficulties may be obviated, and 

 here I would give Mr. Berthon's own description : — " Take or 

 make a box tw^enty inches long and p.bout six inches in its other 

 dimensions, entirely open at one side. Make a hole in one end, 

 and set it upon the otlier. Provide a card to fit the lower end 

 inside ; to the middle of this card attach a narrow slip of black 

 paper (the border of mourning note paper will do), say four 

 inches long. Then place the eye-piece to be measured in the 

 hole, and use my V-dynamometer, and by dividing four inches by 

 the ascertained length of the image of the black line, you have 

 the power of the eye-piece on a focal length of twenty inches, 

 whence its pov/er on any other number of inches may be ascer- 

 tained by simple arithmetic. Let the black slip or mark be 

 pointed at each end, thus -<^BBSaSS0B^B^ and it is surprising 

 how exactly its image can be measured, even to the fi^e-thousandtli 

 of an inch. It is well to have three or four cards with lines of 

 various but exactly mocisured lengths, say two, three, four, etc.. 



