i873«l Notices of Books. 251 



of flint-glass of such curves as would shorten the focal distance 

 (for photographing) to sixty inches. At the proper point, I 

 would place between the two distances an enlarging-lens so con- 

 structed that the normal image of the sun in the principal focus 

 (then about half an inch) would be enlarged to two inches at the 

 distance often inches from the principal focus, viz., at 70 inches 

 from the objective. The camera-box and tube should be one 

 tube, and the focalising rack and screw should be located at the 

 objective end of the tube, thus simplifying the whole arrange- 

 ment and permitting the use of braces, from end to end, to pre- 

 vent flexure ; and on taking off the photographic corrector, and 

 taking out the enlarging-lens, the instrument will be all ready 

 for vision. On consideration, I do not think I would counsel a 

 smaller telescope than the one I have named." 



Professor Newcomb divides the proposed methods of observing 

 the transits of Venus into two classes. The first consists in 

 fixing the moment at which the planet is in contact with the 

 limb of the sun ; the second, in determining the relative position 

 of the centre of the planet and the centre of the sun as often as 

 possible during the transit. The first method, although only 

 that has hitherto been thought practicable, Prof. Newcomb con- 

 ceives liable to inaccuracies ; and he proposes photography as 

 the aid to the second method, in order to form an image of the 

 sun with Venus on its disc, so that points on the plates corre- 

 sponding to the centres of the discs can be fixed with precision, 

 the linear distance between these points being determined by 

 means of a micrometer, and the angle of position obtained from 

 a reference line — this line bearing a relation to the circle of right 

 ascension passing through the sun's centre. For the details of 

 the process of photographing the transit, the corrections neces- 

 sary in the glasses, we must refer the reader to the original 

 papers. 



There are some considerable difficulties connected with this 

 method. The greatest difficulty would appear to be, that the 

 required element appears only as a minute difference between 

 two comparatively long arcs, too long to be measured by a 

 micrometer. But Professor Winlock's apparatus would remove 

 many of the disadvantages. 



These papers contain so much important matter that we hope 

 soon to see the second part. 



Memoirs of the Geological Survey of England and Wales. 

 Vol. iv. : The Geology of the London Basin. Part I. : The 

 Chalk and the Eocene Beds of the Southern and Western 

 Tracts. By William Whitaker, B. A,- (Lond.). (Parts by 

 H. W. Bristow, F.R.S., and T. Mc. K. Hughes, M.A.) 

 London : Longmans, Green, and Co. ; and Stanford. 1872. 

 It is obviously a matter of convenience to the public that the 

 Maps of the Geological Survey, as they are issued sheet by 



