VOL. LXXVII.] PHILOSOPHICAL TRANSACTIONS. i6q 



but influence, and in some respects contribute to the improvement of, our 

 theories in optics. For, admitting that every object-glass or speculum, whose 

 aperture bears less than a certain ratio to its focal length, will begin to give an 

 indistinct picture, it will follow, that while former opticians have been endea- 

 vouring to diminish the aberrations arising from the spherical figure, and the 

 different refrangibility of rays, by increasing the focal length, they have been 

 unaware of exposing themselves to the consequences of the cause of indistinct- 

 ness here pointed out. And till its influence shall be well ascertained and 

 brought to a proper theory, we must suspect that such tables as those which are 

 given in our best authors of optics, pointing out an aperture of less than 6 

 inches for a glass of 120 feet focal length, or a ratio of 1 to 240, must be far 

 from having that degree of perfection which may yet be obtained. No wonder 

 that telescopes, made according to theories or tables, where one of the causes 

 of indistinctness is unsuspected, and therefore left out of the account, can bear 

 no smaller pencil than the 40th or 50th part of an inch ! If then, on one hand, 

 by increasing our apertures we certainly run into great imperfections, we ought 

 nevertheless also to consider what dangers, on the other, we may incur by les- 

 sening them too much. 



As soon as convenient, I intend experimentally to pursue this subject, in 

 order to obtain proper data for submitting this cause of optical imperfection to 

 theory; at present my engagement with the work of a 40-feet reflector will 

 hardly permit so much leisure; and till I shall have repeated, extended, and va- 

 ried these experimental investigations, I would wish them to be considered as mere 

 hints that may afford matter for future disquisitions to the theoretical optician. 



END OF THE SEVENTY-SIXTH VOLUME OP THE ORIGINAL. 



/. An Account of a- New Comet. By Miss Caroline Herschel. Vol. lxxvu. 



Anno 1787. p. 1. 

 The employment of writing down the observations, when my brother uses the 

 20-feet reflector, does not often allow me time to look at the heavens; but as he 

 is now on a visit to Germany, I have taken the opportunity of his absence to 

 sweep in the neighbourhood of the sun, in search of comets; and last night, 

 Aug. 1, 1786, about 10 o'clock, I found an object very much resembling in co- 

 lour and brightness the 27th nebula of the Connoissance des Temps ^ with the 

 dificrence however of being round. I suspected it to be a comet; but a hazi- 

 ness coming on, it was not possible entirely to satisfy myself as to its motion till 

 this evening, Aug. 2. I made several drawings of the stars in the field of view 

 with it, and have inclosed a copy of them, with my observations annexed, that 

 you may compare them together. By the naked eye, the comet is between the 



vol. XVI. Z 



