66 Dr. Herschel on the Power of 



In natural vision, therefore, this power is truly expressed by 

 s/c? I ; and, since we have now also obtained a proper correc- 

 tion x, we must apply it to the incident light with telescopes. 



In the Newtonian and other constructions where two 

 specula are used, there will also be some loss of light on account 

 of the interposition of the small speculum ; therefore, putting b 

 for its diameter, we have A 1 — 6 1 for the real incident light. 

 This being corrected as above, will give the general expression 



va;/ x A x — b 1 for the same power in telescopes. But here we 

 are to take notice, that in refractors, and in telescopes with one 

 reflection, b will be = o, and therefore is to be left out. 



Then, if we put natural light / = 1, and divide by a, we 



have the general form — — — — — for the penetrating power of 



all sorts of telescopes, compared to that of the natural eye as a 

 standard, according to any supposed aperture of the iris, and 

 proportion of light returned by reflection, or transmitted by 

 refraction. 



In the following investigation we shall suppose a = 2 tenths 

 of an inch, as being perhaps nearly the general opening of the 

 iris, in star-light nights, when the eye has been some moderate 

 time in the dark. The value of the corrections for loss of light 

 will stand as has been given before. 



We may now proceed to determine the powers of the instru- 

 ments that have been used in my astronomical observations ; 

 but, as this subject will be best explained by a report of the 



