1 36 Reviews, and. Notices respecting New Books. 



heavens with his 40-feet reflector, with the power of 1000. He 

 supposes that he may obtain 100 favourable hours in the year; then 

 he calculates that to sweep the northern hemisphere, and such part 

 of the southern as is visible, by directing the axis of his telescope 

 into each point of space for a single instant, would require 811 years ; 

 so that his 40-feet telescope may furnish work for his descendants 

 for some generations. Our author concludes this Section by giving 

 a list of the works of Sir W. Herschel. 



The next Section gives an account of a telescope of the Hersche- 

 lian construction, which was erected at Greenwich, a few years 

 since, by a self-taught artist of Aberdeen, of the name of Ramage. 

 In this construction the mechanism is much more simple than that 

 of Sir W. Herschel's great telescope. 



Section XVIII. is taken up with a very curious account of what 

 Sir William Herschel has denominated the space penetrating power 

 of a telescope : we believe he was the first person who considered 

 this subject. This power consists in the most advantageous com- 

 bination of the magnifying power with the quantity of' light. After 

 a very interesting discussion of the subject, he illustrates the differ- 

 ence between this power and the magnifying power by observations 

 of several nebulae and clusters of stars; also of the satellites, of the 

 Georgian planet, and of the Satellites of saturn. 



The Sections commencing with XVIII. and ending with XLVIII. 

 are almost entirely taken up with the important subject of measuring 

 with accuracy small celestial arcs. The first of these Sections gives 

 an historical epitome of the gradual improvements which have taken 

 place in this department of practical astronomy, from the earliest 

 times that we have any account of attempts being made to deter- 

 mine the places of the heavenly bodies, to the present time ; — 

 from the date, when the third part of a degree was only appreciated 

 by estimation, to the present day, when the tenth part of a second 

 may be appreciated, probably with greater certainty. 



The Micrometers, which are described in the following Sections, 

 may be divided into classes, which may again be subdivided as follows: — 



Class I. Single Image Micrometers; containing, 



(1.) Where the measure is taken by means of two parallel spiders 

 lines, the distance of the separation of which can be accurately es- 

 timated : 



(2.) Where the measure is obtained from a divided scale, placed 

 in the common focus of the object- and eye-glasses : 



(3.) By means of reticulated diaphragms, with which the required 

 measure is obtained only by means of a troublesome calculation, 

 deduced from the time of passage. 



Class II. Double Image Micrometers ; containing, 



(1.) The divided Object-glass micrometer: 



(2.) The divided Eye-glass micrometer, with the Catoptric micro- 

 meter, by Ramsden : 



(3.) Prismatic and Cuneiform micrometers : 



(4.) Doubly -refracting Crystal micrometers. 



Class III. Binocular Micrometers. 



