6 
Mr, herschel on the Diameter and 
out in pade-board, increafing by tenths of ail inch from two 
inches to five in diameter, and thefe were made to fit into the 
grooves of the lamp. A good number of pieces, fome of white, 
others of light blue paper, of the fame fize with the pade- 
boards, were alfo cut out, andfeveralof them oiled, to render 
them more tranfparent. The oiled papers fhould be well 
rubbed, that they may not dain the dry papers when placed 
together. This apparatus being ready, we are to place behind 
the pafte board circle, next to the light, one, two, or more, 
either blue or white, dry or oiled, papers ; and by means of 
one or more flames, to obtain an appearance perfectly refem- 
bling the dilk we would compare it with. It will be found, 
that more or lefs altitude of the objeCt, and higher or lower 
powers of the indrument, require a different aflortment of 
papers and lights, which mud by no means be neglected : for 
if any fallacy can be fufpected in the ufe of this apparatus, it 
is in the degree of light we mud look for it. In a few experi- 
ments I tried with thefe lucid difks, where I placed feveral of 
them together, and illuminated them at once, it was found, 
that but very little more light will make a circle appear of the 
fame fize with another, which is one, or even two-tenths of 
an inch lefs in diameter. A well known and driking indance 
of this kind of deception is the moon, jud before or after the 
conjunction, where we may fee how much the luminous part 
of the dilk projects above the red. 
The method of uflng the artificial difks is the fame which 
has been defcribed with the lamp-micrometer, of which this 
apparatus may be called a branch. We are only to obferve, 
that the Planet we would meafure fhould be caufed to go either 
jud under, or jud over, the illuminated circle. It may indeed 
alfo 
