66o SCIENCE PROGRESS 



The Telescope. By Louis Bell, Ph.D., Consulting Engineer; Fellow, 

 American Academy of Arts and Sciences. [Pp. ix + 287, with frontis- 

 piece and 190 figures.] (London : McGraw-Hill Publishing Gd., 1922.) 

 This volume forms a welcome addition to astronomical literature, and will be 

 particularly appreciated by amateurs who possess or are thinking of possessing 

 a telescope of their own. Although there is an extensive literature dealing 

 with the subjects treated by Dr. Bell, it is for the most part scattered and not 

 easily accessible to the average amateur observer. Not only does the book 

 provide interesting reading, but also it contains numerous practical hints 

 which will prove of value to the user of a telescope. 



The first chapter on the evolution of the telescope gives a concise and 

 interesting account of the historical development of the telescope up to the 

 time of the Herschels, from which time the modern telescope may be said 

 to date. The second chapter is concerned with this phase of the subject. 

 The author then deals with optical glass and its working, the properties of 

 objectives and mirrors, mountings, eyepieces, hand-telescopes and accessories. 

 A chapter is devoted to the testing and care of telescopes, in which are given 

 — amongst other things — details of how to test the quality and adjustment of 

 an objective and of the best methods of silvering. Another chapter deals 

 with the setting up and housing the telescope, including particulars of the 

 adjustments required and how to perform them. The last chapter is on seeing 

 and magnification, and is particularly well written, containing just the in- 

 formation which the amateur requires under these headings. 



There are a few points on which comment may be made. On p. 55, in 

 discussing the relative advantages of refractors and reflectors, it is stated that 

 " the reflector as ordinarily proportioned is at a disadvantage, chiefly because 

 it works at F/5 or F/6 instead of at F/15." The statement should surely 

 be inverted : for observation of faint extended objects a large aperture ratio 

 is necessary, and it is a distinct advantage of the reflector that it permits of 

 aperture ratios impossible of attainment in the case of a refractor. 



On p. 190 the statement, in reference to the difiraction grating, that " the 

 spaces between the furrows reflect brilliantly and produce difiraction 

 spectra," is very misleading. In dealing with adjustments in Chapter X, 

 the methods described are of no use for observers near the equator, and 

 alternative methods suitable for equatorial latitudes should be given. On 

 p. 261, the fact that double stars can be resolved beyond Rayleigh's theoretical 

 limit is taken as showing that " the visible diameter of the central disc is in 

 efiect less than the diameter indicated by the difiraction pattern." The cor- 

 rect explanation is that Rayleigh's criterion is based upon an arbitrary 

 assumption which is equivalent to specifying the minimum contrast which 

 the eye can detect. Actually, under favourable conditions a much smaller 

 contrast can be detected. On p. 149, the micro-telescope is stated to be a 

 compact and powerful instrument. The reviewer has looked through many 

 hundreds of these without finding one which could be called even decently 

 good from an optical point of view. 



Particular attention may be drawn to the following paragraph from 

 p. 273 : " With respect to magnifying powers . . . the lowest power which 

 discloses to the eye the detail within the reach of the resolving power of the 

 objective is the most satisfactory." This fact is so often overlooked by 

 amateurs, that it is well to have it stated so explicitly, and for the same reason 

 it is repeated here. H. S. J. 



Oeneral Astronomy. By H. Spencer Jones, M.A., B.Sc. [Pp. viii + 

 392, with 24 plates and 103 illustrations.] (London : Edward Arnold, 

 1922. Price 21S. net.) 



Very few good general textbooks of Astronomy have been written in the 

 English language, and among the best of those that exist none has been 



