472 



facts of interference and diffraction in an experimental and non- 

 mathematical form, This is done with characteristic care and 

 thoroughness. It commences with an explanation of Huygen's 

 principle of wave-motion, and then passes on to consider the 

 diffraction produced by a simple slit, and a lens imaging a point 

 source. In the case of diffraction gratings, the effects of two 

 and then three slits are first discussed, then the cumulative 

 effects of many, in a very ingenious way. Finally, converging 

 illuminating cones are considered. 



An error occurs in Figures 2 and 10. The wave-fronts are 

 shown as having the signs of their curvatures^changed in passing 

 through the substance of the lenses. A slight slip also occurs 

 on p. 6, where it is stated that the intensity of light varies as 

 the square of the amplitude of vibration, but in the next 

 sentence it is stated that the superposition of two vibrations in 

 the same phase, and presumably of the same amplitude, gives 

 rise to light of double intensity. The intensity of light under these 

 circumstances would, of course, be quadrupled. F. J. C. 



Knowledge, 1902. 11^ x 9 in., 288 pages, 12 plates and many 

 figures in the text. London : *' Knowledge " Office, 326, 

 Hiffh Holborn. Price 8s. Qd. 



*&' 



We have much pleasure in calling the attention of those who 

 do not happen to take in this excellent scientific monthly, to the 

 efforts which its editors are now making to cater. for the wants 

 of microscopists. As is probably well known, " Knowledge " was 

 founded by the late Kichard Proctor, and his bias towards 

 Astronomy is reflected in the paper to the present day, although 

 other subjects are by no means neglected, and the whole range of 

 science, from the infinitely great to the infinitely little, seems 

 to be comprised in its scope. During the past year a large 

 number of interesting points in connection with microscopy have 

 been dealt with in the special section conducted by Mr. M. I. 

 Cross, and there have been in addition a number of useful 

 articles on microscopical subjects, such as Collecting and Pre- 

 paring Foraminifera, by Mr. A. Earland ; Preserving and 

 Mounting Rotifers, by Mr. C. F. Rousselet; The Linking 



