32f> PHILOSOPHICAL TRANSACTIONS. [ANNO 1783. 



sures, he employed the lamp-micrometer in preference to the former. The first 

 time he used it on this occasion, he perceived, that if, instead of 2 lucid points, 

 we could have an entire lucid disc to resemble the planet, the measures would 

 certainly be still more complete. The difficulty of dilating and contracting a 

 figure that should always remain a circle, appeared very considerable, though 

 Nature, with her usual simplicity, holds out to us a pattern in the iris of the eye, 

 which, simple as it appears, is not one of the least admirable of her inimitable 

 works. However, he recollected, that it was not absolutely requisite to have 

 every insensible degree of magnitude; since, by changing the distance, he could 

 without much inconvenience make every little intermediate gradation between a 

 set of circles of a proper size, that might be prepared for the purpose. Dr. H. 

 intending to put this design into practice, he contrived the following apparatus. 



A large lantern, of the construction of those small ones that are used with his 

 lamp-micrometer, must have a place for 3 flames in the middle, that we may 

 have the quantity of light required, by lighting 1, 2, or all of them. The 

 grooves, instead of brass sliding doors, must be wide enough to admit a paste- 

 board, and 3 or 4 thicknesses of paper. He prepared a set of circles, cut out 

 in' paste-board, increasing by lOths of an inch, from 2 inches to 5 in diameter, 

 and these were made to fit into the grooves of the lamp. A good number of 

 pieces, some of white, others of light blue paper, of the same size with the 

 pasteboards, were also cut out, and several of them oiled, to render them more 

 transparent. This apparatus being ready, we are to place behind the paste- 

 board circle, next to the light, 1, 2, or more, either blue or white, dry or oiled 

 papers; and by means of one or more flames, to obtain an appearance perfectly- 

 resembling the disc we would compare it with. It will be found, that more or 

 less altitude of the object, and higher or lower powers of the instrument, 

 require a different assortment of papers and lights, which must by no means be 

 neglected: for if any fallacy can be suspected in the use of this apparatus, it is 

 in the degree of light we must look for it. In a few experiments Dr. H. tried 

 with these lucid discs, where he placed several of them together, and illuminated 

 them at once, it was found, that but very little more light will make a circle 

 appear of the same size with another, which is 1, or even 2-tenths of an inch 

 less in diameter. 



The method of using the artificial discs is the same which has been described 

 with the lamp-micrometer, of which this apparatus may be called a branch. We 

 are only to observe, that the planet we would measure should be caused to go 

 either just under, or just over, the illuminated circle. It may indeed also be 

 suffered to pass across it; but in this case, the lights will be so blended together, 

 that we cannot easily form a proper judgment of their magnitudes. The general 

 apparatus employed being now sufficiently explained, several alterations that were 



