540 BIBLIOGRAPHY OF HERSCHEL's WRITINGS. 



Herscliel, "W.: S"iTsroPSis of the Writings of — Continued. 

 A. D. Vol. P. 



494 Class VI No. 1 to No. 19 



496 VII No. 1 to No. 17 



496 VIII No. 1 to No. 40 



498 Notes to special nebulae. 



178G 76 500 Investigation of the cause of that indistinctness of vision tvhich has Jeen 

 ascribed to the smallness of the optic pencil. By William Herschel, 

 LL.D., F. R. S. Read June 22, 1786. 



500 Soon after my first essay of u8iug liigli powers with the Newtonian 



telescope, I began to doubt whether an opinion which has been en- 

 tertained by several eminent authors ' ' that vision will grow indis- 

 tinct when the optic x>encils are less than the fortieth or fiftieth 

 part of an inch " would hold good in all cases. 



I perceived that, according to this criterion, I was not entitled to see 

 distinctly with a power much more than about 320 in a 7-foot tele- 

 scope of an aperture of 6.4 inches; whereas in many experiments 

 I found myself very well pleased with magnifiers which far ex- 

 ceeded such narrow limits. 



This induced me, as it were, by way of apology to myself for seeing 

 well where I ought to have seen less distinctly, to make a few ex- 

 periments. 



501 The first experiments I made were in 1778, and the result of them 



proved so decisive that I have never since resumed the subject, 

 and had it not been for a late conversation with some of my highly 

 esteemed and learned friends, I might probably have left the 

 papers on which these experiments were recorded among the rest 

 of those that are laid aside when they have afforded me the infor- 

 mation I want. 



501 Experiments tvith the naked eye. 



502 Microscopic experiments. 



Exp. 3. With a pencil of ^^ of an inch I saw very distinctly. 



503 Exp. 4. With a pencil oi-rkj of ^t^ inch I saw very distinctly. 

 Exp. 5. With a pencil of xmn of an inch I saw very distinctly. 

 Exp. 8. With a pencil of yt^^^ of an inch I saw very distinctly. 



504 Exp. 10. It occurred to me that a certain proportion of aperture 



might be necessary to a given focal length of an object-glass or 

 speculum. 



505 Exp. 10, 11, 12, 13, show that to see well in microscopes like the one 



used, the aperture of the object glass must bear a considerable 



proportion to its focal length. 

 505 Exp. 15 shows that ^th part of the focal length is not a suflScient 



aperture. 

 507 As soon as convenient I intend to pursue this subject; at present my 



engagement with the work of a 40-feet reflector will hardly permit 



so much leisure, and till I have repeated, extended, and varied 



these experiments, I would wish them to be looked upon as mere 



hints. 



1787 77 4 Remarks on the new Comet. In a letter from William Herschel, LL. 

 D., F. R. S., to Charles Blagden, M. D., Sec. R. S. Read Nov. 

 16, 1786. [Dated Slough, near Windsor, Nov. 15, 1786.] [This 

 comet was discovered by Carolina Herschel, August 1, 1786, 

 during the absence of William Herschel on a visit to Germany, 

 It was comet 1786, II.] 



