224 APPARATUS P0R TELESCOPES; 



XIV. 



befcription oftlie darkening Apparatus for Tele/copes by Means 

 of Fluids. By William Herschel, L. L. D. F. R. S. * 

 with an Engraving. Plate XL 



Conrtru&ion of J\ B F j j j s a f quare trough, clofed at the two oppofite 

 an apparatus for °. ^ ° • * * 



viewing the fun cno *s C D, by well poliflied plain glafles. It will hold any 



through a co- liquid through which the fun's rays are to be tranfmitted. E 

 l«ured fluid. - r „ r , ^ - i *• *< » • c ^ 



is a imall ipout, and F a handle ; lo that any portion ot the 



liquid may be conveniently poured out, when the reft is to be 



diluted. 



The trough is made to fit into the open part of the fkeleton 

 eye tube, Fig. 2. reftirig on the bottom G, and being held 

 in its proper fituation by the fides H and I, the end K at the 

 time of obfervation is put into a fhort tube fixed to the New- 

 tonian telefcope, and jnay be turned about fo as always to 

 have the open part H I horizontal. 



When the eye-piece Fig. 3. is fcrewed by its end M into 

 the fkeleton tube at L, Fig. 2. and the trough, Fig. 1 . with 

 any liquid to be tried is placed in the open part G H I, the 

 fun's rays will come from the fmall mirror of the telefcope to 

 K, and patting through the plain glafles C D, inclofing the li- 

 quid, wjll enter the eye-piece M, and after the neceflary re- 

 fractions come to the eye at N« Any other tingle or double 

 eye-piece of different magnifying powers may be fcrewed into 

 L, inftead of the piece Fig. 3 ; and the liquid may eafily be 

 tempered fo as to intercept a proper quantity of light to fuit 

 every eye-glafs which is in ufe, and thus to render the inflec- 

 tion of the fan perfectly convenient. 



* Philof. Tranf. 1801, p. 3*2, mentioned at page 21 of our 

 Journal, but in that Number there was no room for the Plate. 



XV. On 



