1 34 Reviews, and Notices respecting New Books. 



upon which the dome revolves will at once accommodate themselves 

 to a trifling shrinking of the parts. We imagine that the author's 

 objection to the light copper covering might be obviated, by having 

 an interior covering (not in contact with the copper) made of some 

 material that is a bad conductor of heat : perhaps oii-cloth might 

 be found effectual for this purpose. 



From §. IV. to §. VII. inclusive we find an interesting and in- 

 structive account of the most important instrument used in practical 

 astronomy, the Refracting Telescope. The account of this 

 instrument given in these sections is extremely valuable, for the 

 practical instructions contained in them, as to certain requisites in 

 determining the goodness of a telescope. 



In §. IV., Art. 9 — 14, our author details the results of his own 

 experience, in the examination of the figure of an object-glass. The 

 rest of this and the two following Sections are dedicated to the de- 

 scription of the celestial eye-piece, from the single lens, to the most 

 effective combination, the eye-pieces of Huygens and Kamsden, — 

 the first denominated the negative eje-piece, where the image is 

 formed behind the second lens of the combination, and the latter 

 the positive eye-piece, the image being formed before it. This last 

 is the eye-piece used in telescopes furnished with the spider's lines. 

 Both these constructions are found to diminish the aberration ari- 

 sing from sphericity, and also from the unequal refrangibility of 

 light. For determining the power of the telescope our author has 

 given a dioptric formula, for the focal length of a single equivalent 

 lens, expressed in terms of the focal lengths of the combined lenses 

 and the distance between their centres ; which formula contains the 

 principle of the celestial eye- piece with variable magnifying powers. 



The next Section contains an account of diagonal or reflecting 

 eye-pieces to be used for high altitudes; but our limits will not allow 

 us to enter further into details. Section VI. contains an account 

 of the celestial eye-piece with variable powers, with a small Table of 

 powers constructed for one of the author's telescopes. In §. VII. 

 we have an account of the erect or terrestrial eye-piece, with its 

 various modifications and improvements. 



In §. VIII. we have an explanation of the principle of Achro- 

 matism. We extract from this section an interesting statement 

 relating to the actual construction of an object-glass, according to 

 Sir John Herschel's computations, in his paper "on the Aberration 

 of Compound Lenses and Object-glasses." 



" An object-glass having three convex surfaces, and only one con- 

 cave, was constructed by Tulley from Sir John Herschel's compu- 

 tations; and though the formation of the flint lens was very different 

 from what he had been accustomed to execute in his ordinary 

 practice, yet he succeeded in accomplishing his object. The object- 

 glass turned out so good, that it was competent to separate double 

 stars oi' the first class, and to exhibit minute objects very distinctly 

 with an aperture of 3*^5 inches, and a focal length of 45*0." 



The next Section is appropriated to the description of various kinds 

 of stands for refracting telescopes. It is of the greatest importance 



