30 



SCIENCE 



[N. S. Vol. XXXIV. No. 862 



Some Characteristics of Light-negative Selenvum: 



Miss LiLAH B. Crum. 

 The Use of a Ballistic Galvanometer and a Pen- 

 dulum for Measuring rapidly Fluctuating Re- 

 sistances: Wm. H. Clark. 



Many methods for measuring constant resist- 

 ances are employed. The method here described is 

 one that we have used in the physical laboratory 

 at the State University of Iowa to measure a 

 fluctuating resistance. The apparatus consists of 

 a pendulum, a Wheatstone bridge mesh including 

 a battery of small E.M.F., and a galvanometer. 

 The unknown resistance in the fourth arm of the 

 bridge in this particular case happened to be a 

 selenium cell. The pendulum was about thirty 

 inches in length and swung through an are of 

 twenty-eight inches. The arc over which it swung 

 was graduated with respect to time, the smallest 

 division being .00125 sec. The period of vibra- 

 tion was .6 sec. Four keys adjusted to open and 

 close two electrical circuits were placed on the arc 

 over which the pendulum svning. The operation 

 of the first two keys illuminated the selenium cell, 

 the second two keys operated the galvanometer 

 circuit. The length of time that either circuit was 

 closed was determined by the distance between the 

 two keys which opened and closed that circuit. 

 Keys fci and Tc., which operated the lighting circuit 

 were placed a given distance apart corresponding 

 to the desired length of exposure of the cell to 

 light. Then keys fcs and K were placed apart a 

 constant distance equal to .05 sec. If the change 

 of resistance is called As and the length of ex- 

 posure to light is called At, then Ax^cd/At 

 where d is the deflection and c is the galvanometer 

 constant. This equation is used for small resist- 

 ances. When As becomes comparable to x another 

 device is employed. Since Aa; is a function of the 

 deflection we replace the unknown fluctuating 

 resistance by a known variable resistance and, 

 keeping the ratio arms of the bridge the same, 

 vary the resistance to secure deflections covering 

 the same range of scale. A curve between change 

 of resistance and deflection is plotted. Then for 

 any deflection caused by the fluctuation of the 

 unknown resistance we can at once read from this 

 curve the corresponding change of resistance. The 

 method is probably as accurate and as easy to 

 manipulate as any method that has been devised 

 for measuring rapidly fluctuating resistances. 



The Nature of Light-action in Selenium: F. C. 

 Brown. 



The Doppler Effect in Electrodeless Discharge: 

 Frank F. Almt. 



Some Laboratory Apparatus in Elementary Phys- 

 ical Measurements: Frank F. Almy. 



The Action of Epinephrim upon the Muscle Tissue 

 of the Vein: John McClintock. 



A Method for Studying Embryos as Belated to 

 Medical Work: H. J. Prentiss. 



The Peripheral Distriiution of Cranial Nerves in 

 ^lecturus maculatus: H. W. Norris and Mar- 

 garet Buckley. 



The Zoological EanTc of Necturus as Indicated iy 

 the Origin and Distribution of its Cranial 

 Herves: H. W. Norris. 



Notes on Methods for the Study of Amphibian 

 Eggs and Larvoe: Albert Kuntz. 



The Development of Lymph Channels in Turtles 

 by the Fusion of Mesenchymal Spaces: Frank 

 A. Stromsten. 



1. The theory that the lymphatic system is 

 budded off from the venous system is a direct 

 product of a special method of investigation, i. e., 

 the injection method. 



2. The injection method alone is entirely unre- 

 liable, because: (o) Only that portion of the 

 lymphatic system which is in direct connection 

 with the point of injection (lymph sacs) is shovpn, 

 the unconnected spaces are not indicated. (6) 

 Extravasations and venous injections vitiate the 

 results. 



3. Serial sections, both injected and iminjected, 

 of turtle embryos of different ages show the suc- 

 cessive stages of the development of lymphatics 

 from the ' spongy mesenchyme surrounding the 

 aorta and larger arteries, through the formation 

 of independent spaces which constantly enlarge 

 and finally fuse to form continuous channels. 



4. The endothelium of the lymphatics arises en- 

 tirely independent of the venous endothelium from 

 the original mesenchymal cells. 



Some Notes on Iowa Meptiles: M. P. Somes. 

 Biiilding a Museum: T. Van Hyning. 



An outline plan of building an Iowa state mu- 

 seum; showing the approximate amount of mu- 

 seum material in the state required for a museum, 

 together with showing the number of cases re- 

 quired, case space, floor space, etc., for a com- 

 pleted museum representative of the state. A co- 

 operative plan of school museums, whereby the 

 schools of the state may cooperate with the state 

 museum in supplying a museum for all of the 

 schools is proposed. 



Notes on the Blach-Crowned Heron : B. H. Bailey. 

 The College Museum: B. H. Bailey. 



