458 Annbjses of Booh. [JuNis, 



divided olject glass micrometer. In the old micrometer of this 

 construction, invented by Savery, two semi-lenses were made to 

 separate from, and approach to, each other by a fine screw ; and 

 when tlie two images of the object were in contact, the disrunce of 

 the centres of the semi-lenses was a measure of the angle which it 

 subtended. In Dr. Brewster's micrometer, however, the semi- 

 lenses, fixed at an invariable distance, are made to move between 

 the object glass and its focus, so that the two images can easily be 

 brought into contact, and the angle measured upon a scale of equal 

 parts as large as the focal length of the object glass. 



The luminous range micrometer, which is entirely a new instru- 

 ment, is intended to measure the angle subtended by two luminous 

 objects. By pushing in the eye-piece the two luminous points are 

 swelled into circular images of light ; and when these images touch 

 one another, their angular distance is indicated npon a scale of equal 

 parts. 



For an account of the other micrometers, namely, the circular 

 mother-of-pearl micrometer, the rotatory micrometer, and the eye- 

 piece micrometer, we must refer the reader to the work itself. 



In the last chapter of this book the author discusses the subject of 

 micromctrical fibres, and proposes to astronomers to employ glass 

 fibres, for tlie purpose of removing the error arising fioni the in- 

 flexion of light. 



The instruments described in Book II. cannot easily be explained 

 without figures. The most important of them is a reflecting genio- 

 meter for measuring the angles of crystals, and a new angular wire 

 micrometer. The rejiecling geniometer is an instrument of very 

 extensive use, not merely in measuring the angles of crystals, but 

 in all optical experiments where the angles of prisms require to be 

 ascertained. The angular wire micrometer is intended for the same 

 purposes as the position micrometer iuventod by Dr. Herschel, 

 whicii consists of two wires, one fixed, and the other moveable, 

 crossing one another in the centre of the field, and forming every 

 angle from to 180°. In this construction it is very easy to read 

 off the angle formed by the two wires ; but it is obvious that all the 

 observations must be made on one side of the centre of the field ; 

 that the real scale of the instrument is measured by the semidia- 

 meter of the field ; and that in the case where the vertex of the 

 angle is without the field, it is incapable of measuring it. 



In order to get rid of all these defects, Dr. B. places both the 

 fixed and the moveable wire at a little distance from the centre of 

 the field, so that their intersection is variable. By this contrivance 

 a longer radius is obtained in the mensuration of the angle, and the 

 observations may be made near the very centre of the field ; but 

 while these advantages are gained, there is apparently a great difli- 

 culty in judging the value of the angle upon the scale. By the aid 

 of a beautiful property of the circle, in which the angle formed by 

 two lines placed in a circle is equal to half the sum, or half the 

 difference of the arches which they intercept according as the lines 



