BIBLIOGRAPHY OF IIERSCHEL's WRITINGS. ~)G1 



Herschel, W.; Synopsis ok tiii-. WinriNOS of— Continued. 



A. D. Vol. P. 



tbiit power siiOicieutly to sIr-w by tbo dial of a distant church 

 steepki what o'clock it was, notwithstanding tiio naked cyo couhl 

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

 ing power ; for though it might recpiire 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 38.91). 



1800 yO 68-81 [Dimensiousof variousteJescopesusedbyHEUSCllEL, and their calcu- 

 lated si)ace-penetrating powers are given, toget her with a largo num- 

 ber of examples quoted from his note-books illustrative of the rela- 

 tions of magnifying and space-peuetratiug powers.] 



82 Comparison of the space-penetrating power of 20-foot telescope calcu- 



lated according to tho 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 ()12 times as far as Sirins, can any 

 longer be seen in this telescope. 



83 The space-penetrating ]>ower of the large re tlec tor was 192; admit- 



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

 this telescope would show stars of the 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 tho 40- 

 Ibot telescope, assuming 100 hours of observing time in the year as 

 the most probaljle deduction from experience, is 812 years. 

 •[Dated] Slough, near Windsor, Jnue 20, 1799. 



1800 90 255 Fiircsiigatioii of tlic Powers of ilie pri.sm<i(ie Colours to heal and illuminate Ob- 

 jects; with remarks that prove the difcreHtPcfrangihilitii of radiant Heat, 

 lb tvliich is added an Inquiri/ into the Method of vicwinfj the Sun advan- 

 tageoiishj tvitli Telescopes of lartjc Apertures and high magnifyinei Powers. 

 By William Herschel, LL. D., F. R. S. Read March 27, 1800. 



255 " It is sometimes of great use in natural philosophy to doubt of things 

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

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

 our reach. 



2.5r) "In a variety of experiments I have occasionally made relating to tho 

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

 tage, I used various combinations of dill'erently 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 scarcer any sensation of heat. Now, 

 as in these different combinat ions 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 aresult, we have the proportion of the rising of the thermometer iu 

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



