362 SUMMARY OF CURRENT RESEARCHES RELATING TO 



relative phases are altered by shifting the grating, and the image pattern 

 in the view-field is altered in consequence. A simple case is considered 

 in the paper, and it is proved that the image in the view-plane may 

 change without an alteration in the position of the diffracted images. 



Light Waves and their Uses.* — Although this work is of little 

 assistance to us in solving those intricate microscopical problems which 

 depend on the wave theory of light, it is nevertheless a most fascinat- 

 ing book. More than this, it will be useful to those of our Fellows, 

 who, not having studied physical optics, are desirous to understand 

 something about the fundamental principles by which the Microscope 

 image is formed. 



The drawback to all books on this subject is, that if they are worth 

 anything at all, they are crammed full of mathematics, and, therefore, 

 intelligible only to those acquainted with that form of hieroglyphic 

 writing ; if, on the other hand, the treatment is popular, they are 

 generally so inaccurate as to be valueless. 



Here we have a book by one of the highest authorities, written 

 with hardly a mathematical symbol, and in a peculiarly pleasant style. 



The author, A. A. Michelson, speaking of wave-motion, says that 

 it " is one of the most fascinating, not only of the department of 

 science, but of human knowledge. If a poet could at the same time 

 be a physicist, he might convey to others the pleasure, the satisfaction, 

 almost the reverence, which the subject inspires." 



The chapter succeeding that upon Wave Motion and Interference, 

 deals with the Resolving Limits for the eye, the Telescope and the Micro- 

 scope. It is shown that the resolving power of the eye is about ^^ in. 

 at 10 in., and that this amount is augmented five-hundredfold by a 5 in. 

 telescope and four-hundredfold by a Microscope. 



The larger part of the book is occupied by showing how quantities 

 (linear and angular), far smaller than any that can be dealt with either 

 by the telescope or Microscope, can be measured by means of the 

 Interferometer — an instrument, as its name suggests, for measuring 

 interference phenomena. 



Those who have studied mathematical instruments will be much 

 interested in the description of the harmonic analyser, as well as in the 

 woodcut showing some of the wonderful curves it has drawn. 



Some idea of the accuracy gained in measurements made by the 

 Interferometer may be obtained from the following example. 



A standard metre, measured by three different observers at different 

 times, separated by whole months, was found to be equal in length to 

 310678 '48 red waves of light. The greatest difference between the 

 three measurements was only one-fifth of a single wave-length. 



There are three plates giving excellent coloured representations of 

 spectra, and the book is well illustrated by woodcuts throughout. 



Simple Method for the Observation of Ultra-Microscopic Par- 

 ticles.! — E. S. London demonstrated to the Microbiological Society of 

 Petersburg (November 8, 1903) a new apparatus of Siedentopf and 



* Decennial Publications of the University of Chicago. University of Chicago 

 Press, 1903, 8vo, 166 pp , 3 pis. and 108 figs. 



f Centralbl. Bakt., Eef., xxxiv. (1904) pp. 433-4. 



