112 



The advantages of this species of micrometer are: the 

 large scale, the line images formed, and the facility of mea- 

 suring diameters in every possible direction. Its imper- 

 fections are: that, to different eyes, and under different 

 circumstances of the same eye, the length of the focal di- 

 stance, that suits distinct vision, Avill vary; and, of course, 

 tlie quantity of the measures, given by the scale, are liable 

 to a small variation. The goodness of the telescope is, 

 also, in some degree, impaired, by the application of this 

 contrivance of a divided object-glass. 



It should be noted, that' the wire and object-glass mi- 

 crometers, were both adapted, in their turns, to the same 

 achromatic telescope; and the comparative observations 

 made as near to each other, in point of time, as possible. 



The diameters of the sun, measured by the ten-inch sex- 

 tant, were taken with a small achromatic telescope, mag- 

 nifying about twelve times, and were observed on the limb, 

 and on the arch of excess, several times alternately; the 

 measures being always finished in the same direction of 

 the micrometer's screw: and the quarter of the double mea- 

 sure was used as the semidiameter, with the addition of 3"; 

 Avhich is the known diminution of the image of the sun's 

 semidiameter, after the reflections and refractions it under- 

 goes in the process. " As the three kinds of micrometers, 

 just described, are so completely different from each other, 

 in their construction, adjustment, and mode of mensuration, 

 I consider them as fully sufficient to make an experiment 

 on the probable consistency of the results which may be 



obtained 



