in Optical Instruments. 25 
I now proceed to the next part of my subject, which is the con- 
sideration of the Newtonian telescope. This admirable instrument, 
such as could only be expected from the genius of the immortal 
philosopher who invented it, has, (as a necessary consequence of 
its construction,) among its other valuable peculiarities, that of 
having less false light in it than any other kind of telescope.—The 
same striking effect, therefore, will not be manifest in excluding 
the trifling fog there is in it, as in another construction where it is 
more abundant. Nevertheless there is something to be done.—If 
we examine the pencil of rays proceeding from its eye-piece with 
the magnifier, it does not precisely represent the image of aspangle 
or a piece of black cloth, as it should do ;—a good deal of foreign 
light may be seen, formed partly by the side of the tube behind the 
diagonal metal, and partly by such portion of the end next the 
large mirror, as the plain one can reflect along with the image, toge- 
ther perhaps with some reverberated by the little tube which carries 
the magnifiers. 
In Fig. II. is represented the method I haye taken to remedy 
these imperfections in a 77-inch focus, and seven-inch aperture 
Newtonian*. The alterations from the common construction, are 
* On exhibiting these alterations to those celebrated artists, Messrs. Tulley of 
Islington, I learnt from them that they had lately made an arrangement of the 
same description, in a Newtonian telescope, made for Mr. Camfield of North- 
ampton. These gentlemen (whose unrivalled pre-eminence in their profession, 
needs not my feeble testimony, or eulogium) fully admit that the exclusion of the 
false light, makes a great difference in the performance of the telescope, in the 
day-time, but do not seem to think any alteration is to be perceived in viewing 
celestial objects. Iam aware, that I am paying myself an indifferent compli- 
ment in differing from such authorities. Ihave given my reasons for so doing, 
and cannot help still being of opinion, that it is scarcely possible to select any 
object in the heavens, and to view it without rays from a variety of others also 
finding their way into the telescope, and thus disturbing the singleness of vi- 
sion, which would exist was there but one star or object in the heavens to emit 
light. I think, in particular, that the double ring of Saturn, and its belts and 
shadows are not perfectly seen, unless the telescope employed to view them, 
will show black objects perfectly black, and white objects perfectly white, and of 
course all the intermediate gradations of shade correctly ; very few telescopes 
will shew the division between the rings of Saturn quite black, (as mine does,) 
owing to the false light so generally prevalent in them, 
