BIBLIOGRAPHY OF HERSCHEl's WRITINGS. ' 567 



Herschel, W.: Synopsis of tmh Wkitixgs of— Coutinucd. 



A. D. Vol. p. 



that power sufficiently to shew by the dial of a distant church 

 steeple what o'clock it was, uot withstanding the naked eye could 

 no longer see the steeple itself. Here I only speak of the penetrat- 

 ing power ; for though it might require magnifying power to see tho 

 figures on the dial, it could require none to see the steeple." 

 The space-penetrating power of this telescope was 33.99. 



1800 90 G8-81 [Dimensionsofvarioustelescopesuscdby Herschel, and their calcu- 

 lated space-penetrating powers are given, together wit h a large num- 

 ber of examples (luoted from his note-books illustrative of tho rela- 

 tions of maguifyiug and space-penetrating powers.] 

 8-2 Comparison of the space-penetrating power of 20-foot telescope calcu- 

 lated according to the principles developed in this paper with that 

 deduced from observation, as given in Phil. Trans., vol. 75, p. 247, 

 248. A substantial agreement leads to the conclusion that no single 

 star above 489.551, or at most 612 times as far as Siriits, can any 

 longer be seen in this telescope. 

 83 The space-penetrating power of the large reflector was 192; admit- 

 ting that stars of the 7th magnitude are visible to the unassisted eye, 

 this telescope would show stars of tho l,342d magnitude. There- 

 fore, a cluster of 5,000 stars might be seen by the 40-foot reflector at 

 a distance at least 300,000 times that of the nearest fixed star. 

 84-85 The calculated time necessary to sweep t he whole heavens with the 40- 

 foot telescope, assuming 100 liours of ol)serving time in the year as 

 the most i)robal)le deduction from experience, is 812 years. 

 [Dated] Slough, near Windsor, June 20, 1799. 



1800 90 255 Investigation of the Poivcrsoftheprismatic Colours to heat and illuminate Ob- 

 jects ; with remarJxs that prove the <liff( rent Eefraufiibili li/ of radiant Heat. 

 To which is added an Ineiuirij into the Method of vicwinr/ the Sun advan- 

 tatjeoushj with Telescopes o/ lareje Apertures and high magnifijing Powers. 

 By William Hehschel, LL. D., F. K. S. Read March 27, 1800. 



255 "If is sometimes of great usi^ in natural philosophy to doubt of things 

 that are commonly taken for granted ; especially as the means of 

 resolving any doubt, when once if is entertained, are often within 

 our reach. 



25G " In a variety of experiments I have occasionally made relating to the 

 method of viewing the sun, with large telescopes, to the best advan- 

 tage, I used various combinations of differently coloured darkening 

 glasses. What appeared remarkable was that when I used some of 

 them I felt a sensation of heat, though I had but little light ; while 

 others gave me much light, with scarce any sensation of heat. Now, 

 as in these different combinations the sun's image was also differently 

 coloured, it occurred to me that the prismatic rays might have the 

 power of heating bodies very unequally distributed among them; and 

 as I judged it right in this respect to entertain a doubt, it appeared 

 equally proper to admit the same with regard to light. If certain 

 colours should be more apt to occasion heat, others might, on the 

 contrary, be more fit for vision, by possessing a superior illuminat- 

 ing power. At all events, it would be proper to recur to experi- 

 ments for a decision." 



256-2G0 Eight experiments to test heating power of red, green, and violet 

 regions of the prismatic spectrum. 



201 As a result, we have the proportion of the rising of the thermometer in 

 red to that in green as more than 2J to 1 ; in red to violet about 3^ to 1. 



