256 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777. 



each in llieir turn, for tlie more comtnodious measurement of different angles. 

 Thus it may be very convenient to use one set of prisms for measuring angles 

 not exceeding 36', and consetjuently fit for measuring the diameters of the sun 

 and moon, and the lucid parts and distances of the cusps in their eclipses; and 

 another set of prisms to measure angles not much exceeding 1 minute, and con- 

 sequently fit for measuring the diameters of all tlie other planets. This latter 

 set of prisms will be the more convenient for measuring small angles, on account 

 of a small imperfection attending the use of this micrometer, as before men- 

 tioned; namely, that angles cannot be measured with it when the prisms ap- 

 proach very near the focus of the object-glass, the pencils of rays being there 

 lost at the point where the prisms touch each other. 



On the principles that have been here explained, a prism placed within the 

 telescope of an astronomical instrument, adjusted by a plumb-line or level, to 

 receive all the rays that pass through the object-glass, may conveniently serve 

 the purpose of a micrometer, and supersede the use both of the vernier scale 

 and the external micrometer; and the instrument may then be always set to 

 some even division before the observation. Thus the use of a telescopic level 

 may be extended to measure, with great accuracy, the horizontal refractions, 

 the depression of the horizon of the sea, and small altitudes and depressions of 

 land objects. Time and experience will doubtless suggest many other useful ap- 

 plications of this instrument. 



A paper from the learned Abbe Boscovich was read before this Society the Qth 

 of last June, describing a similar contrivance as an invention of the Abbe Rochon, 

 in which the Abbe Boscovich himself also claims some share; I therefore desire 

 to acquaint this Society, that I communicated this invention to Mr. Dollond, 

 and had it executed by him ; and also showed the instrument itself, so executed, 

 to my esteemed friend Alexander Aubert, Esq., fellow of this Society, a gentle- 

 man very well qualified to judge of things of this nature, above a year before 

 the communication of the Abbe Boscovich's paper, as will appear from their 

 written attestations, drawn up at my desire, describing the particulars of the 

 communication of this invention which I made to them so long ago. Mav I be 

 permitted to remark, that this instrument having been executed by my directions, 

 in several forms, by Mr. Dollond, between the months of March and August, 

 1776, and set up and trierl at his house in the presence of several of his work- 

 men, could not be considered as an absolute secret concealed from the public. 

 However, I doubt not that the following attestations of Mr. Aubert and Mr. 

 Dollond, will sufficiently prove my title to this invention of the prismatic micro- 

 meter; and I take this opportunity of exhibiting to the Society the instrument 

 itself, mentioned in Mr. Dollond's letter as executed by himself according to my 

 directions, and sent to the Royal Observatory in the month of August 177(i. 



Greenuic/i, Dec. 11, 1777. 



