Mr. Herschel on Reflecting Telescopes. 289 



of the superb equatorial, constructed by him for the Observatory 

 at Dorpat. The existence of such an instrument in such hands 

 ought to be a matter of congratulation to every lover of science, 

 and I am rejoiced to hear from Mr. Fraunhofer, that, so far from 

 resting satisfied with what he has already achieved, he has ex- 

 tended his views to the construction of still more powerful instru- 

 ments, and is actually employed on an equatorial of 12 inches 

 aperture and 18 feet focal length, which will indeed deserve to be 

 regarded as a chef d'ceuvre of art. 



I have not the least disposition to doubt the merits of the Dor- 

 pat telescope, which I have not seen, but which I am disposed to 

 believe one of the best, probably the very best refracting tele- 

 scope, ever made. The accounts which Mr. Struve has been so 

 kind as to transmit me of its extraordinary power are quite sa- 

 tisfactory. A telescope capable of easily separating the stars of 

 u Leonis, and giving exact and coincident measures of their rela- 

 tive position on several nights, can admit of no question as to its 

 excellence. I have hitherto heard but of one other refracting tele- 

 scope in which this star has been seen double. It is that by 

 Lerebours, now at the Royal Observatory at Paris, the object glass 

 of which (like the Dorpat telescope) is 9 inches in aperture, though 

 only Sj are effectively employed. Mr. Struve, however, has dis- 

 covered double stars still closer than u Leonis with his magnificent 

 instrument. 



My immediate object in writing this, however, is to obviate an 

 erroneous impression which may arise in the minds of those who 

 read Mr. Fraunhofer's memoir, as to the great inferiority of reflect- 

 ing telescopes in point of optical power, to achromatics in general, 

 and more especially to those constructed with such delicacy as his 

 own doubtless are. Those who have witnessed the performance 

 of Mr. Amici's beautiful Newtonian reflectors, will not readily 

 admit this inferiority, but will rather feel disposed to wish that 

 some attempt might be made to accommodate such admirable in- 

 struments to the more exact purposes of astronomy, an object 

 which appears to hfave been too easily lost sight of. 



Mr. Fraunhofer's expressions, when speaking of the loss of light 



