314 



SCIENCE. 



[N. S. Vol. XXI. No. 530, 



ington and in answer to legitimate inquiries 

 by mail. 



Under these circumstances it seems that the 

 facilities now offered by the Library of Con- 

 gress meet the need indicated in Dr. Sumner's 

 letter to a very considerable extent, and fur- 

 ther advances in this direction will occur if 

 it appears that valuable service can be ren- 

 dered. 



I conclude by inviting the readers of Science 

 to make use of these new facilities whenever 

 the library resources to which they have access 

 are inadequate to the needs of the investiga- 

 tions which they have in hand. Communica- 

 tions should be addressed to the Librarian of 

 Congress, and should be marked ' Science 

 Section ' if they are inquiries referring to the 

 mathematical, physical or natural sciences. 



J. David Thompson. 



THE STORAGE OF MICROSCOPIC SLIDES. 



To THE Editor of Science: In your issue 

 of December 30 you published an article by 

 C. L. Marlatt, of the U. S. Department of 

 Agriculture, describing a method of storing 

 and indexing microscopic slides. 



The Bausch and Lomb Optical Company 

 have designed and are selling an excellent 

 cabinet with card system which has all the 

 advantages claimed by Mr. Marlatt for his 

 and lacking only the envelopes, which I can 

 not but think must be somewhat inconvenient. 



These cabinets are made in three sizes, hold- 

 ing 500, 1,500 and 3,000 slides respectively. 

 Tiers of trays, each running in its own groove, 

 are constructed to take slides of various sizes. 

 At the bottom are drawers (one, two or three) 

 containing separate cards for every slide, on 

 each of which is printed a form for register- 

 ing the slide: Tray No. — Series No. — Name 

 of Slide — Stain — Mounted in — and two lines 

 for other data. There are also printed guide 

 cards from A to Z. 



The objects being recorded on separate 

 cards, the removal of slides necessitates 

 simply the removal of its corresponding card, 

 while the addition of slides requires only the 

 filling out and insertion of new cards. Classi- 

 fication thus, it will be seen, becomes exceed- 

 ingly simple. The slides may be rearranged 



and the collection increased or diminished 

 with the least possible amount of trouble. 



Josephine Shatz. 



Rochester, N. Y., 

 January 8, 1905. 



SPECIAL ARTICLES. 

 doppler's principle and light-beats. 



There is a beautiful lecture experiment in 

 illustration of Doppler's principle due, I be- 

 lieve, to Koenig. A vibrating tuning fork 

 of high pitch, say 2,000 vibrations per second, 

 is moved to and fro near, and at right angles 

 to, a reflecting wall. The waves coming from 

 the fork and (virtually) from its image back 

 of the wall are changed in length by the op- 

 posite motions of fork and image with the 

 result that very audible beats are heard. With 

 a fork of the pitch mentioned, a speed of 

 three feet per second gives beats at the rate 

 of about eleven per second. Although special 

 forks are made for this experiment, they are 

 quite unnecessary. An ordinary C 512 fork 

 of Koenig's pattern gives a very shrill tone 

 when strongly bowed near the shank and 

 answers the purpose admirably. If the fork 

 is held stationary and the reflecting surface 

 is moved, the effect is the same on account of 

 the motion of the fork's image. 



Attempts to secure visible beats by means 

 of light waves of slightly different wave-length 

 have met with no success, partly on account 

 of rapid changes of phase, and partly because 

 of the difficulty of securing two sources whose 

 vibration frequencies are nearly enough equal. 

 Thus if we assume (what is most likely not 

 true) that the failure to observe interference 

 fringes with differences of path greater than, 

 say, 30 cm. indicates a change of phase, this 

 would indicate 10° or more changes of phase 

 per second. On the other hand, should we 

 take the two D lines as sources there would 

 be about 10" beats per second. It is evidently 

 almost hopeless to attempt to secure visible 

 light-beats in this manner. If we consider 

 Doppler's effect, however, the case is quite 

 otherwise. The second form of Koenig's ex- 

 periment, viz., that in which the reflector is 

 moved, is in principle almost exactly anal- 

 ogous to Professor Michelson's interferometer. 



