VOL. LXI.] PHILOSOPHICAL TRANSACTIONS. 205 



arid Declination, 7vith Dollond's Micrometer; with other New Applications of 

 the same. By the Rev. N. Mashelyne, B. D., F. R. S., &fc. p. 536. 



The divided object glass micrometer, as happily applied by the late Mr. John 

 Dollond to the object end of a reflecting telescojje, and now with equal advan- 

 tage adapted by the present Mr. Dollond, his son, to the end of an acromatic 

 telescope, is so easy of use, and affords so large a scale, that it is generally con- 

 sidered by astronomers as the most convenient and exact instrument for mea- 

 suring small distances in the heavens. But, as the common wire micrometer is 

 peculiarly adapted for measuring differences of right ascension and declination 

 of celestial objects, and is not near so convenient or exact for measuring their 

 absolute distances; so on the contrary the object glass micrometer is peculiarly 

 fitted for measuring distances, and has generally been supposed incapable of or 

 unfit for measuring distances of right ascension and declination. Thus the 2 

 micrometers, as mutually supplying each other's defects, have been esteemed both 

 equally necessary in their turn to be used by the practical astronomer, and con- 

 sequently to have a place in every well-furnished observatory. Every astronomer, 

 who has time and inclination for making a variety of observations, would 

 undoubtedly wish to be supplied with, and to make use of both. But, as every 

 person desirous of making observations for his own amusement, or public utility, 

 may not happen actually to be furnished with, nor chuse to be at the expence of pro- 

 viding himself with both, it is certainly a very desirable thing, if he could be 

 enabled to make that use of the instrument he has, which might supply, in some 

 measure at least, the want of the other which he has not. Therefore, as the 

 object glass micrometer may be applied with little trouble, and but small 

 additional expence, to the measuring differences of right ascension and declination, 

 with an exactness little, if at all, inferior to what they can be obtained with the 

 common micrometer, Mr. M. gives here the directions necessary to be followed 

 when it is used in this manner. He afterwards shows how differences of right 

 ascension and declination between the limbs of the sun and Venus or Mercury, 

 and distances of the limbs both in lines parallel and perpendicular to the 

 equator, may also be observed in the transits of these planets over the sun. 



A small addition will be necessary to be made to the apparatus of the object 

 glass micrometer, to enable it to answer these purposes, viz. a cell, containing 2 

 wires intersecting each other at right angles, placed in the focus of the eye-glass 

 of the telescope, and moveable round about, by turning a button. Let enws, 

 pi. 5. fig. 9, represent the field-bar of the telescope, ew and ns two wires inter- 

 secting each other at right angles at c, and moveable about the same as a centre, 

 in manner above-mentioned. vSuppose it be required to measure the difference 

 of right ascension and declination of two stars, whose difference of declination 

 does not exceed the extent of the scale of the micrometer, and the distance of 



