72 T>r. Herschel on the Power oj 



" I can see that it is a cluster of stars, many of them being 

 " visible." 



If we compare the penetrating power of the two instruments, 



we find that we have in the first ,4 ' x 2 42 ' lz% = 12,84 '■> and 

 in the latter — dgiLilr^! = 20,25. However, the magnify- 

 ing power was partly concerned in this instance ; for, in the 

 sweeper it was not sufficient to separate the stars properly. 



March 4, 1783. With a 7-feet reflector, I viewed the nebula 

 near the 5th Serpentis, discovered by Mr. Messier, in 1764. 



" It has several stars in it ; they are however so small that I 

 *° can but just perceive some, and suspect others." 



May 31, 1783. The same nebula with a 10-feet reflector; 



penetrating power — iilii-i — 1— == 28,67. 



" With a magnifying power of 250, it is all resolved into 

 " stars : they are very close, and the appearance is beautiful. 

 " With 600, perfectly resolved. There is a considerable star not 

 " far from the middle ; another not far from one side, but out 

 " of the cluster; another pretty bright one; and a great number 

 " of small ones." 



Here we have a case where the penetrating power of 20 fell 

 short, when 29 resolved the nebula completely. This object 

 requires also great magnifying power to shew the stars of it 

 well ; -but that power had before been tried, in the 7-feet, as far 

 as 460, without success, and could only give an indication of its 

 being composed of stars ; whereas the lower magnifying power 

 of 250, with a greater penetrating power, in the 10-feet instru- 

 ment, resolved the whole nebula into stars. 



