penetrating into Space by Telescopes. 69 



its theory has not been inquired into. This undoubtedly gave 

 rise to the invention of those very useful short telescopes called 

 night-glasses. When the darkness of the evening curtails the 

 natural penetrating power, they come in very seasonably, to 

 the relief of mariners that are on the look out for objects which 

 it is their interest to discover. Night-glasses, such as they are 

 now generally made, will have a power of penetrating six or 

 seven times farther into space than the natural eye. For, by 

 the construction of the double eye-glass, these telescopes will 

 magnify 7 or 8 times ; and the object glass being 2^ inches in 

 diameter, the breadth of the optic pencil will be 3-i or 3-i tenths 

 of an inch. As this cannot enter the eye, on a supposition of 

 an opening of the iris of 2 tenths, we are obliged to increase the 

 value of a, in order to make the telescope have its proper effect. 

 Now, whether nature will admit of such an enlargement becomes 

 an object of experiment ; but, at all events, a cannot be assumed 



less than . Then, if x be taken as has been determined for 



m 



three refractions, we shall have — '-12*1L— -— q^q or y^g. 



Soon after the discovery of the Georgian planet, a very cele- 

 brated observer of the heavens, who has added considerably to 

 our number of telescopic comets and nebulae, expressed his wish, 

 in a letter to me, to know by what method I had been led to 

 suspect this object not to be a star, like others of the same 

 appearance. I have no doubt but that the instrument through 

 which this astronomer generally looked out for comets, had a 

 penetrating power much more than sufficient to shew the new 

 planet, since even the natural eye will reach it. But here we 

 have an instance of the great difference in the effect of the two 

 sorts of powers of telescopes ; for, on account of the smallness 



