VOL. LXVn.^ PHILOSOPHICAL TRANSACTIONS. 25/ 



To the Rev. Dr. Mashelyne. 

 REV. SIR, St. Paul's Church-yard, Nov. 22, 1777. 



According to your desire I send the following particulars of the experiments 

 which were made by your directions, for completing a new kind of micrometer 

 for measuring small angles. About the beginning of April J 776, I received 

 your first directions respecting this matter, which were to make 2 prismatic glasses 

 or wedges, of such angles that rays of light, which passed through them, should 

 be refracted about 18' of a degree: these were to be placed between the object- 

 glass and eye-glass of an achromatic telescope about 30 inches long. The an- 

 gular edges of the 1 prismatic glasses were to be placed in contact with each 

 other; they were to be moved in a parallel position from the object-glass to the 

 focus of the eye-glass, and to be of such a size as to cover the aperture of the 

 object-glass when brought close to it. By the refraction of these wedges 2 

 images were formed in the telescope, which were at the greatest distance, about 

 36, when the wedges were close to the object-glass, and approached as they were 

 moved towards its focus, where they united; so that the whole focal distance of 

 the object-glass was to be the length of the scale for measuring the angular dis- 

 tance of the 2 images formed in the telescope. When these wedges were ap- 

 plied, as above described, the 2 images were found to be coloured to a great 

 degree, occasioned by the refraction of the wedges. This defect you directed me 

 to remove, by making the prismatic glasses or wedges achromatic, on the same 

 principles as the achromatic object-glasses; and, after some difficulties, this was 

 effected; the 2 images formed in the telescope appeared free from colours and 

 distinct. The above experiments were made in a rough wooden tube, with an 

 inconvenient method of moving the wedges by hand; in this state it was when 

 shown to Alexander Aubert, Esq., f.r.s., towards the end of May, 1776; after 

 which you desired to have it done in a more complete manner, in a brass tube, 

 with a means of turning the tube round to take angles in different directions, 

 and a method of moving the wedges with a screw. This was completed about 

 the middle of August in the same year, and then sent to the Royal Observatory. 

 I have the honour to be, &c. Peter Dollond. 



I hereby certify, that in the month of May, 1776, the Rev. Mr. Maskelyne, 

 Astronomer-royal, produced to me, at Mr. Dollond's house in St. Paul's 

 Church-yard, and in his presence, as a new invention of his own, an instrument 

 for measuring small angles, consisting of 1 achromatic prisms or wedges applied 

 between the object-glass and eye-glass of an achromatic telescope about 30 inches 

 long, by moving of which wedges nearer to, or further from, the object-glass, 

 the two images of an object produced by them appeared to approach to, or recede 

 from, each other, so that the focal length of the object-glass became a scale 

 for measuring the angular distance of the two images. 



London, Nov. 27, 1777. ALEX. AUBERT. 



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