396 Scientific Intelligence. 



disc connected with a pendulum and to serve as a terminal to 

 deliver the spark to the moving disc. With this apparatus he 

 finds that he can follow changes of potential under twenty volts. 

 He finds that the electrostatic capacity of the bobbin of the induc- 

 tion coil is of the order 16X10 -12 farad. The working of the 

 iron core of the coil diminished with increasing number of vibra- 

 tions. — Wied. Ann., No. 3, 1899, pp. 535-562. j. t. 



8. Electric waves on wires. — W. D. Coolidge has succeeded 

 in photographing electric waves on wires by using a vigorous 

 exciter — Blondlot, or Tesla transformer. The photographic lens 

 was placed above the wires, which were horizontal. The nodes 

 and ventral segments are clearly distinguished in the photographs. 

 — Wied. Ann., No. 3, 1899, pp. 578-591. j. t. 



9. Absorption of the X-rays by air. — Since very short ultra- 

 violet waves of light are absorbed by air, it is an interesting 

 question to consider the absorption of the X-rays by air. Some 

 recent experiments by Prof. Trowbridge and Mr. J. E. Burbank 

 in the Jefferson Physical Laboratory instituted to study this 

 question show that the difference of absorption of a column of air 

 thirty inches in length at atmospheric pressure and a column of 

 air of the same length at four millimeters pressure cannot be dis- 

 tinguished by the use of a fluoroscope. With cathode rays the 

 difference is as thirty-three to one. That is, the absorption is 

 thirty-three times as much in air at atmospheric pressure as at 

 four millimeters pressure. The electrostatic effects in the ex- 

 hausted tube thirty inches long are very beautiful and cease 

 when the concav.e mirror of the X-ray tube becomes the positive 

 terminal. j. t. 



10. Die Optischen Instrumente der Firme R. Fuess-; deren 

 Beschreibung, Justierimg und Anwendung von C. Leiss; pp. xiv, 

 397, with 233 text figures and three photo-plates. Leipzig, 1899 

 (Wm. Engelmann). — Very important advances have been made 

 within recent years in the methods and apparatus applicable to 

 crystallographic and optical investigations. The present volume, 

 giving an account of the instruments furnished by the firm of R. 

 Fuess (Berlin), is very thorough and complete and will be of 

 great value to all working in this field. There are described here, 

 lor example, a number of forms of spectrometers and various 

 types of apparatus to be used with them, as, for example, the new 

 spectral arrangement of Wiilfing, giving homogeneous light of 

 different wave lengths. The reflectometers, spectrographic appa- 

 ratus, goniometers, microscopes, are also fully and minutely 

 treated of. The chapter giving an account of the different forms 

 of apparatus for projections with electrical lamps is particularly 

 interesting and timely. The volume is not simply a catalogue of 

 instruments, but gives full descriptions of the method of use applic- 

 able in each case, with references to the original papers. The 

 worker is^ thus told not only what instruments are to be had, but 

 when he is supplied with them, how they are to be adjusted and 

 used in practical investigations. 



