Abstract of a Register of the Barometer, Thermometer, and Rain, at 

 Lyndon, in Rutland, for the year 1798. By Thomas Barker, Esq, 

 Read Dec. 12, 1799. [PhiL Trans. 1800, p. 46.] 



On the Power of penetrating into Space by Telescopes ; with a compa- 

 rative Determination of the Extent of that Power in natural Vision, 

 and in. Telescopes of various Sizes and Constructions ; illustrated by 

 select Observations. By William Herschel, LL.D. F.R.S. Read 

 Nov. 21, 1799. [Phil. Trans. 1800, p. 49.] 



It has long been observed that the power of distinguishing objects 

 at great distances depends not only on the magnifying power applied 

 to the telescope through which they are viewed, but also on the quan- 

 tity of light emitted by the object, and collected and conveyed to the 

 eye by means of the instrument. The superiority of telescopes with 

 large apertures must hence appear obvious ; and we have long wit- 

 nessed the essential improvements made in this respect by Dr. Hers- 

 chel, which have enabled him to extend his view into the firmament 

 to distances, the bare mention of which is sufficient to astonish a mind 

 unaccustomed to investigations of this nature. That it is principally 

 the increased quantity of light that enables us to view luminous ob- 

 jects, at great distances will appear manifest if we reflect that, since 

 the density of light decreases in the ratio of the squares of the di- 

 stances of the objects emitting the light, it follows that an object may 

 be removed to distances at which its light will be so rarefied as to 

 produce no longer any sensation upon the optic nerve : that if an 

 optical instrument be used with an object-glass of a larger diameter 

 than the pupil of the eye, the quantity of light collected by this means 

 in the eye will be greater in proportion to the greater extent of the 

 object-glass compared with that of the pupil : and that hence the 

 most distant star that can be seen with the naked eye, if it be viewed 

 through a tube with an object-glass of twice the diameter of the pupil, 

 it will without any magnifying power be visible at a distance four times 

 greater than that at which the naked eye ceased to perceive it. Dr. 

 Herschel many years ago adverted to this circumstance, when in his 

 paper on the Construction of the Heavens, he introduced what he then 

 figuratively called his sounding line, to which he now substitutes the 

 appellation of the power of penetrating into space. And in the present 

 paper he fully investigates a comparative determination of the extent 

 of that power in natural vision, and in telescopes of various sizes and 

 constructions; all which he illustrates by a number of select and cu- 

 rious observations. 



In the first part of the paper he establishes the difference between 

 magnifying and penetrating powers ; he rejects some vague terms in 

 common acceptation, to which he substitutes algebraic symbols and 

 such accurate definitions as enable him to proceed upon solid ground. 

 And after distinguishing between self-luminous objects and those 

 which shine by a reflected light, and likewise noticing those whose 



