MICROMETER MEASURES. 25 



principally this : that, in adjusting the telescope, and the mi- its scale deter- 

 crometer wires, to distinct vision, no alteration is made, by "^^ned. 

 the difference of the conformation of the eye, or of focal dis- 

 stance, tfcat suits that of the observer, in the value of the arc 

 to be measured. The principal defects of it are : the diffi- 

 culty of judging accurately of bisections, or contacts of the 

 fine wires, by the limbs to be measured ; and the impossibilt y 

 of observing any diameter, except the one perpendicular to 

 the equator. 

 The obiect-fflass micrometer is an instrument, now so fami- DoUand's ob- 

 ■> >=> , • 1 • ject glass mi- 



liar to every person conversant m the use or astronomical m- crometer. 



struments, that it is only necessary to say, that mine was 

 made, and adapted to a triple object-glass achromatic tele- 

 scope, of 4-2 inches focal distance, by Mr. DoUand, and its 

 scale very carefully verified by himself; and that the scale is, 

 as usual, divided into inches, lOths, 20ths, and vernier divi- 

 sions : that^ when it is applied, it lengthens the focal ditance 

 of the telescope about 6 inches : thus making it 48 inches, or 

 4 feet focal distance. 



The advantages of this species of micrometer are : the its advantages. 

 large scale, the fine images formed, and the facility of measur- 

 ing diameters in every possible direction. Its imperfections 

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

 of the same eye, the length of the focal distance, that suits 

 distinct vision, will vary ; and, of course, the quantity of the 

 measures, given by the scale, are liable to a small variation. 

 The goodness of the telescope is, also, in some degree, im- 

 paired, by the application of this contrivance of a divided 

 object-glass. 



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

 ters, were both adapted, in their turns, to the same achromatic 

 telescope; arftUhe comparative observations made as near to 

 each other, in point of time, as possible. 



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

 were taken with a small achromatic telescope, magnifying tant of Trough- 

 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 directiortof the micrometer-screw: 

 and the quarter of the double measure was used as the semi- 

 diameter, with the addition of 3" ; which is the known dimi- 

 nution of the image of the sun's sen^idiameter, after the refleo- 



