Septembee 21, 1900.] 



SCIENCE. 



445 



search was obtained. The results were classified 

 and an order of standing was determined under 

 the four heads : reliability and simplicity ; lia- 

 bility to act when not required ; ease of oper- 

 ation ; cost of equipment and maintenance. 



The most remarkably wide range of prices 

 is reported— $30 to $585, averaging about $200. 

 Eleven of the list tested are authorized for use, 

 and the Board determined that the common 

 form of brake now in use should be replaced 

 by one or another of these, or equally efficient, 

 brakes. 



President Vreeland, of the Metropolitan Co., 

 his directors and the executive ofScers seem 

 ^to have taken much trouble and to have met 

 most of the expenses of these important pioneer 

 investigations, and his electrical engineer, the 

 master mechanic and the superintendents lent 

 essential aid in the work. 



The report can be had by applying to the 

 Board at Albany. 



The data may be summarized thus : At 8 

 miles an hour, a stop was made in from 3 to 8 

 seconds ; at 12 miles in 5 to 9 seconds ; at 15 

 miles, in 6 to 10 seconds ; at 16 miles, in 6 to 

 11 seconds, without sand, and 6J to 9J with 

 sand. The distances run ranged from 35 to 66 

 feet at 8 miles, 58 to 111 at 12 miles, 72 to 203 

 at 16 miles ; averaging for all speeds, from 58 

 to 133 feet. 



A conventional system of checking for ' skid- 

 ding ' wheels was adopted. 



All in all, the work must be accepted as an 

 earnest and faithful endeavor to effect, for the 

 first time, a solution of an important problem — 

 one which concerns all railway managements 

 and all travellers on electric street cars very 

 seriously. The report has been criticised as 

 failing to give data relating to dimensions of 

 parts, uncertainty regarding the comparability 

 of brakes differently handled by their exhibit- 

 ors, and regarding the automatic records. An 

 examination of the apparatus employed, how- 

 ever, shows that the distances traversed were 

 measured by a mechanism positively driven 

 and which, therefore, gave reliable compari- 

 son of distances traversed, which measures are 

 the sole basis of all comparisons and are evi- 

 dently substantially correct. The technical 

 journals generally approve the report as giving 



valuable and helpful information. Undoubt- 

 edly, later investigations will afford opportuni- 

 ties for improvements which this, as all pio- 

 neer efforts, indicates to be desirable notwith- 

 standing its evident and admitted defects in 

 time-measurement, the report must be accepted 

 as important. Variations of the time-scale do 

 not affect its conclusions. It is to be hoped that 

 the work will be continued and perfected. 

 R. H. Thukston. 



Lehrhuch der Photochromie von Wilhelm Zenker; 



neu herausgegeben. Von Peofessoe De. B. 



SCHWALBE. Braunschweig, Friedrich Vie- 



weg & Sohn. 



This is a republication of a work which ap- 

 peared in 1868, to which has been added a bio- 

 graphical sketch, and a r6sum6 of recent work 

 along similar lines. 



It will doubtless surprise the general reader 

 to find that partially successful experiments in 

 photochromy, or the direct reproduction of color 

 by photography, were made over a quarter of 

 a century before the announcement of Da- 

 guerre's discovery in 1839. As early as 1810 

 Seebeck obtained colored impressions of the 

 solar spectrum on paper coated with chloride of 

 silver, but the matter attracted but little at- 

 tention and was soon forgotten. 



In 1841 the property which this substance 

 possessed of assuming a color somewhat similar 

 to the hue of the light falling upon it was re- 

 discovered by Herschel, but the possible great 

 importance of the subject does not appear to 

 have been realized until Becquerel, stimulated 

 by Daguerre's discovery, took up the work, and 

 by a laborious series of investigations deter- 

 mined the conditions most suitable for a faithful 

 reproduction of the colors of the original. 



Up to the time of the appearance of Zenker's 

 work the almost universal opinion seems to 

 have been that colored compounds of silver 

 (oxidation and reduction products) were formed 

 by the action of the light. Zenker, however, 

 offered a most ingenious physical explanation, 

 as opposed to the chemical theory. He ex- 

 plained the colors as due to the interference of 

 light reflected from thin laminse of metallic 

 silver, laid down in sheets half a wave length 

 apart, by the action of stationary light waves, 



