84 & r - Herschel on the Power of 



comes out to be above n| millions of millions of millions of 

 miles ! A number which exceeds the distance of the nearest 

 fixed star, at least three hundred thousand times. 



From the above considerations it follows, that the range for 

 observing, with a telescope such as my 40-feet reflector, is 

 indeed very extensive. We have the inside of a sphere to exa- 

 mine, the radius of which is the immense distance just now 

 assigned to be within the reach of the penetration of our instru- 

 ments, and of which all the celestial objects visible to the eye, 

 put together, form as it were but the kernel, while all the im- 

 mensity of its thick shell is reserved for the telescope. 



It follows, in the next place, that much time must be required 

 for going through so extensive a range. The method of exa- 

 mining the heavens, by sweeping over space, instead of looking 

 merely at places that are known to contain objects, is the only 

 one that can be useful for discoveries. 



In order therefore to calculate how long a time it must take 

 to sweep the heavens, as far as they are within the reach of my 

 4,0-feet telescope, charged with a magnifying power of 1000, 

 I have had recourse to my journals, to find how many favour- 

 able hours we may annually hope for in this climate. It is to 

 be noticed, that the nights must be very clear ; the moon 

 absent ; no twilight ; no haziness ; no violent wind ; and no 

 sudden change of temperature; then also, short intervals for 

 filling up broken sweeps will occasion delays ; and, under all 

 these circumstances, it appears that a year which will afford 90, 

 or at most 100 hours, is to be called very productive. 



