HERSCHEL. 211 



(marked ii by Flamstced and i in Harris's maps,)* tlie star tliat pre- 

 cedes a Orionis w of the same constellation, and you will then be pre- 

 pared for the more difficult observation of tj Coroiiai. Indeed, rj Coronse 

 is a sort of miniature of i Bootis, which may itself be considered as :x 

 miniature of « Geminorum.''" (Philosophical Transactions, 1782, p. 100.) 



As soon as Piazzi, Gibers, and Harding had discovered three of the 

 numerous telescopic })lanets now known, Herschel proposed to himself 

 to determine their real magnitudes; but telescopes not having then been 

 applied to the measurement of excessively small angles, it became requi- 

 site, in order to avoid any illusion, to try some experiments adapted to 

 giving a scale of the powers of those instruments. Of the labor of oar 

 indefatigable astronomer in this line, I am going to give a condensed 

 account. 



The author relates first, that in 1774 he endeavored to ascertain ex- 

 Ijerimentally, with the naked eye, and at the distance of distinct vision, 

 what angle a circle must subtend to be distinguished by its form from a 

 square of similar dimensions. The angle was never smaller than 2' 17"; 

 therefore, at its maximum it was about one-fourteenth of the augle sub- 

 tended by the diameter of the moon. 



Herschel did not say of what nature the circles and squares of 

 paper were that he used, nor on what background they were projected. 

 It is an omission to be regretted, since in those phenomena the intensity 

 of light must be an important feature. However it may have been, the 

 scrupulous observer, not daring to extend to telescopic vision what 

 he had discovered relative to vision with the naked eye, he undertook to 

 do away with all doubt by direct observations. 



On examining some heads of pins, placed at a distance in the open 

 air, with a three-foot telescope, Herschel could easily discern that those 

 bodies were round when the subtended angles became, after being 

 magnified, 2' 19'^ This is almost exactly the result obtained with the 

 naked eye. 



When the globules were darker — when, instead of pins' heads, small 

 globules of sealing wax were used — their spherical form did not begin to 

 be distinctly visible till the moment when the subtended magnified 

 angles — that is, the moment when the natural angle multiplied by the 

 magnifying power — amounted to five minutes. 



In a subsequent series of experiments, some globules of silver, placed 

 very far from the observer, allowed their globular form to be perceived, 

 even when the magnified angle remained below two minutes. 



* In the selection of i Bootis as a test, Arago has taken the precantiou of fiiviug its 

 corresponding denomination in other catalogues, and Bailey appends the following note, 

 No. 20o2, to 44 Bootis : " In the British Catalogue this star is not denoted by any letter, 

 hut Bayer calls it r, and on referring to the earliest 3iIS. catalogue in MSS. Vol. XXV, 

 I find it is there so designated ; I have therefore restored the letter." (See Bailey's 

 edition of Flamsteed's British Catalogue of Stars, 1835.) The distance between the 

 two members of this double star is 3" .7, and position 23°.5. (See Bedford Cycle.) — 

 Translator. 



