38 



SCIENCE 



[N. S. Vol. XXIX. No. 731 



Magnification also depends, of course, upon 

 the distance of mirror from the lens. 



The apparatus has been especially helpful in 

 making rapidly, and to scale, accurate draw- 

 ings of insect wings, mouth-parts and the like. 

 One student, who was also working in verte- 

 brate zoology, found it of the greatest service 

 in making from negatives enlarged drawings 

 of the arrangement and distribution of the 

 scales of various reptiles. But in addition to 

 this outline work it is also perfectly feasible 

 to use it with more detailed drawings from 

 microscopic preparations if the room be well 

 darkened. It is quite possible that a similar 

 device has long been in use by others, but I 

 have failed to find any mention of it and I 

 have, therefore, thought that this description 

 might be of aid to some. 



Wm. a. Eiley 



SOCIETIES AND ACADEMIES 



THE PHILOSOPHICAL SOCIETY OF WASHINGTON 



The 655th meeting was held November 21, 1908, 

 President Bauer in the chair. Three papers were 

 read at this meeting, as follows: 

 The Radiation Laios of Metals: W. W. Coblentz. 



A knowledge of the laws governing the radiation 

 of metals with variation in temperature is of 

 interest in connection with the numerous specula- 

 tions offered to explain the great light emissivity, 

 i. e., the high luminous efficiency of the new in- 

 candescent lamps with metal filaments. 



The speaker described some results obtained in 

 an investigation of the most important so-called 

 constant, o, of the type which obtains for a 

 " black body " in which a — 1 = 4, in Stefan's 

 law of total radiation. The substances examined 

 were tungsten, tantalum, osmium, platinum and 

 several types of carbon filaments, usually in the 

 form of 110-volt incandescent lamps. The spectral 

 distribution of energy was measured with a bolom- 

 eter. 



It was found that - the so-called constant, a, 

 decreased with rise in temperature, and in all 

 cases higher in value than that of platinum. 



It seems to be a phj'sical property of metals to 

 have a low reflecting power in the visible, and 

 especially in the ultra-violet, part of the spectrum. 

 Throughout the infra-red the reflecting power of 

 metals is uniformly high. The low reflecting 

 power in the visible spectrum causes an abnor- 

 mally high emission in this region, which, in con- 



nection with the high values of the so-called 

 constant, o, accounts for the high luminous effi- 

 ciency. 



Although it seems to have been overlooked 

 heretofore, it is obvious that the so-called con- 

 stant, a, must decrease in value and approach 

 that of a " black body," otherwise a point would 

 be attainable at which the radiation is greater 

 than that of a black body at the same tem- 

 perature. 



From the results obtained it is evident that the 

 spectral emissivity function of metals must be far 

 more complex than that given in the Wien equa- 

 tion. Since the emissivity is a function of the 

 reflecting power, which is a function of the re- 

 fractive index and of the absorptive coefficient, 

 which, in turn, are functions of the temperature, 

 the wave-length and the electrical conductivity, 

 it is evident that the spectral emissivity equations 

 must contain factors which will take account of 

 these phenomena. 

 Determination of Flexure of Pendulum Supports 



by the Interferometer : W. H. Buegee. 



In measuring the force of gravity by means of 

 a swinging pendulum, the observations are neces- 

 sarily made under varying conditions, and the 

 period of vibration of the pendulum is conse- 

 quently afl:ected by many causes, and corrections 

 have to be applied before the value of g can be 

 ascertained. One of the important corrections 

 results from the elastic yielding of the pendulum 

 support. Several methods have been used to 

 measure the flexure of the pendulum support and 

 to ascertain its effect upon the period of the 

 oscillating pendulum. Each of these methods 

 contains some doubtful assumptions, and to avoid 

 which the new plan of employing the interfer- 

 ometer for measuring the absolute displacement 

 due to flexure of pendulum support was devised 

 by Messrs. Hayford and Fischer, of the Coast and 

 Geodetic Survey. The instrument used is a modi- 

 fied form of the Michelson interferometer. In 

 observing, the instrument is separated into two 

 parts, the main body of the instrument, and the 

 mirror attached to the pendulum case, each being 

 carried on entirely separate supports. 



The experiments carried out by the speaker 

 included tests of flexure of both pendulum case 

 and of the pier upon which the case was mounted. 

 The displacements were found to be movements of 

 rotation. For comparison the static force method 

 was also used in the experiments. Measurements 

 of displacement were made with the pendulum 

 case mounted with various substances between the 



