41 i renetntktt lnh Space hy Tele/copes* 



On the Power of penetrating into Space by Tekfiopes ; with a comparative Determination of 



the Extent of that Power in natural Vifion., and in Tele/copes of various Sizes and Con- 



JtruSions; illuflrated byfelcEh Obfervations. By William Hekschel, LL. D. F. R. S.* 



I 



T will not be difficult to fliew that the power of penetrating into fpace by telefcopes is 

 very different from magnifying power, and that, in the conftrudtion of inftruments, thefe 

 two powers ought to be confidered feparately. 



In order to conduft our prefent inquiry properly, it will be neceffary to examine the 

 nature of luminous bodies, and to enter into the method of vifion at a diftance. There- 

 fore, to prevent the inaccuracy that would unavoidably arife from the ufe of terms in their 

 common acceptation, I {hall have recourfe to algebraic fymbols, and to fuch definitions as 

 may be neceffary to fix a precife meaning to fome expreffions which are often ufed in con- 

 verfation, without much regard to accuracy. 



By luminous bodies I mean, in the following pages, to denote fuch as throw out light 

 whatever may be the caufe of it : even thofe that are opaque, when they are in a fituation 

 to refleft light, fiiould be underftood to be included j as objeds of vifion they muft throw 

 out light, and become intitled to be called luminous. However, thofe that fliine by their 

 own light may be called felf-luminous, when there is an occaGon to diftinguifti them. 



The queftion will arife, whether luminous bodies fcatter light in all directions equally } 

 but, till we aire more intimately acquainted with the powers which emit and refled light, 

 we fiiall probably remain ignorant on this head. 1 ftiould remark, that what I mean to fay, 

 relates only to the phyfical points into which we may conceive the futfaces of luminous 

 bodies to be divided ; for, when we take any given luminous body in its whole cohftruc- 

 tion, fuch as the fun or the moon, the queftion will affume another form, as will appear 

 hereafter. 



That light, flame, and luminous gafes are penetrable to the rays of light, we know from 

 experience ; f it follows therefore, that every part of the fun's difk cannot appear equally 

 luminous to an obferver in a given fituation, on account of the unequal depth of its lumi- 



* Philofophical Tranfaftions, iSoo, p. 49. 

 t In order to put this to a proof, I placed four candles behind a fcreen, at | of an inch diftance from 

 each other, fo that their flames might range exaftly in a line. The firftof the candles was placed at the 

 fame diftance from the fcreen, and juft oppofite a narrow flit, two-thirds of an inch long, and J broad. On 

 the other fide of the fcreen I fixed up a book, at fuch a diftance from the flit that, when the firft of the 

 candles was lighted, the letters might not be fulficiently illuminated to become legible. Then, lighting " 

 fucceflive!> the fecond, third, and fourth candles, I found the letters gradually more illuminated, fo that 

 at laft I could read them with great facility; and, by the arrangement of the fcreen and candles, the light 

 of the fecond, third, and fourth, could not reach the book, without penetrating the flames of thofe that 

 were placed before them, 



nous 



