Dr. Pearson's Introduction to Practical Astronomy. 137 



Class IV. The Verniers. 



Class V. The Reading Microscopes. 



Sections XIX. and XX. are taken up with the description of the 

 Spiders-line Micrometer, under its most improved form, as constructed 

 by Troughton. The author explains in these Sections, how to 

 obtain the value of the micrometer screw, &c. &c. 



He has described in §. XXIII. an instrument of his own, called 

 a Polymetric Reticle. We have no doubt that for many purposes 

 this micrometer is as perfect as any that requires illumination. 



We refer to §. XXII. for the means of making the spider's lines 

 or the divisions visible in the dark. 



The next method of measuring small arcs, which depends on a 

 single image, is the reticulated diaphragm. The account of the va- 

 rious diaphragms, with the method of registering and reducing the 

 observations as given by La Caille and Smeaton, are contained in 

 the Sections from XXIV. to XXVII. inclusive. 



We come in §. XXVIII. to the second class of micrometers, viz. 

 the Double Image Micrometers, which we must pass over with the 

 same brevity as the others. 



Section LXXXVI. contains the accourft of another Double Image 

 Micrometer, by Amid, which our author had not seen when this 

 part of the work was sent to the press. When Dr. Maskelyne was dis- 

 satisfied with the performance of the divided object-glass, he sug- 

 gested another plan, the account of which is given in §. XXXV. 

 It consisted in applying two thin prisms instead of the divided lens, 

 for measuring the sun's or moon's diameter j and the great advantage 

 gained was, that both the images were equally bright in all parts 

 of the lengthened scale. Dr. M., however, dying before he had 

 matured his plan, it seems never to have been made much use of. 

 But our author discovered how to avail himself of this principle for 

 measuring small angles. He has called this the Cuneiform Micrometer; 

 and for the account of this construction we refer to §. XXXVI. 



We now come to the application of the doubly-refracting crystal 

 to the construction of a double image micrometer. The account of 

 the application of this curious substance, to the purposes of micro- 

 metrical measurements, is contained in four Sections. 



After giving a complete account of Sir David Brewster's micro- 

 metrical telescope, the author goes on to the description of the 

 third class of micrometers, viz. Binocular Micrometers : this is con- 

 tained in the next two Sections. The first of these gives an ac- 

 count of a Lamp Micrometer, invented and used by SirW.Herschel. 

 We would particularly direct our readers to an attentive perusal of 

 Section XLIV. as containing valuable directions for the use of Posi- 

 tion Micrometers. 



The next kind of micrometer is that which is independent of the 

 telescope, the Vernier. But before this, or the reading microscope 

 can be made use of, it is necessary to be enabled to give a sloxv and 

 steady motion to the radius carrying the vernier on the limb of the 

 instrument, or to the limb itself. Therefore before our author enters 



Third Series. Vol. 3. No. 14. Aug. 1833. T 



