Febbuart 23, 1883. 



SCIENCE. 



73 



miliar sources of light is of interest. The angle /3 is 

 the azimuth of the jSficol's xDrism. 



SOHKCE. /3 



Petroleum flame 71.1° 



Illuminaling gas (argand = ordinary Ijurner) 71.5 



Lime liglit 76.7 



IncandesceDt electric lamp (near maximum of brightness) , 77.8 



Arc light about 79.0 



Magne.=iium ligbt 86.3 



Buriligbt • SO. 5 



( Wied. ann., 1SS2, 990.) c. s. h. [184 



Diffraction in telescopes. — A paper on tlie 

 effect of diffraction on the appearance of a bright 

 disk of indefinitely great radius as seen in a tele- 

 scope, by H. Struve. — ( IKied. ami., 1882, 1008.) 

 c. s H. [185 



Polarization of diffracted light.— The investi- 

 gation here described relates to the modification 

 which plane polarized light undergoes in diffraction 

 by a reflecting grating of glass, of collodion, or of 

 speculum metal. It is thus closely allied to Friih- 

 lich's research, though of a more general character. 

 The author, W. Konig, found that within the range 

 of deviation, where elliptical polarization was marked, 

 the determinations of azimuth were not very satisfac- 

 tory : hence attention was given chiefly to difference 

 of phase in the two components. This difference 

 was measured by a Babinet conapensator. All of the 

 results were in satisfactory accordance with Eelhy's 

 theory of splierical polarized wave-surfaces, by which 

 he explained the phenomena observed by Frohlich. 

 The experiments go far to reconcile the contradictory 

 results, obtained by experimenters, who, following 

 Stokes, have attempted thus to determine the relation 

 of the plane of polarization to that of vibration; but 

 at the same time Eethy's theory seems to end all 

 hope of deciding this interesting point by the most 

 promising means hitherto suggested. — (Wied. arm., 

 1882,1016.) c. s. H. [186 



Elliptic double refraction. — E. Lommel de- 

 velops his theory of refraction, to apply to the case 

 of propagation of light-waves in a medium which 

 rotates the plane of polarization. The equations 

 yield a form of Blot's law for rotation involving the 

 index of refraction , which corresponds well with obser- 

 vation. — {CarVs repert., xviii. 673.) c. s. H. [187 



Galileo's telescope. — An extended discussion of 

 the theory of this form of telescope, by W. Pscheidl. 



— {Carl's repert., xviii. 686.) c. s. H. [188 



{Photography. ) 

 Photography as applied to animal locomotion. 



— A simple method of studying photographically the 

 movements of animals is described by M. G. Demeny. 

 In front of the camera is placed a rapidly revolving 

 disk, containing a narrow sectorial window. A white 

 animal is selected, which moves in the sun befoi-e a 

 very black background, best an opening in a dark- 

 ened shed. The exposures with sensitive plates may 

 be reduced to the to'ittt part of a second, the interven- 

 ing intervals being sufficiently long so that the images 

 shall not be superposed. By knowing the rate of 

 the disk, the speed of the animal may be measured 

 from the negative. If the plate is caused to move 

 in the opposite direction to the image of the animal, 

 the exposures may be made more frequently without 

 fear of superposition, as has been done by M. Marey 

 inhis 'photogi-aphicsun' (La Nature, April 22, 1882). 

 By having a number of windows in the disk, the 

 course of small, rapidly moving objects may be stud- 

 ied: for example, the trajectory of a white stone 

 thrown from the hand, or a white paper attached to 

 the circumference of a carriage-wheel. — (Journ. de 

 j3/i2/s., Nov., 1882.) w. H. P. [189 



Production of Iotw temperatures. — ^ After com- 

 paring the various methods for producing low tem- 

 peratures, Mr. Rawbothara concludes that the method 

 l3y the evaporation of ammonia is the best ; ammonia 

 being jsreferable to other liquids, chiefly on account 

 of its high latent heat, and high pressiu-e at low tem- 

 peratures. — (Journ. Frankl. hist., ixxxv. 2.) c. B. P. 



[190 



Heat of solution and of dilution of perchloric 

 acid. — In his researches on the oxyacids of chlorine, 

 M. Berthelot has been conducted to the study of the 

 heat of formation of perchloric acid. The solution 

 of the liquid monohydrated acid in one hundred times 

 its weight of water at 19° sets free -f20.3 cal. This 

 enormous heat, which exceeds that of all the common 

 monohydrated acids, explains the extreme difference 

 which exists between the action of this acid in soltt- 

 tion, and the action of the monohydrated acid. It is 

 found that the molecular specific heats of solutions 

 of perchloric acid, between 40° and 15°, can be repre- 

 sented by the formula, — 



„ , 273.8 742.2 



n being greater than 6. 



The heats of dilution of the acid when in different 

 degrees of solution can be represented by a peculiar 

 hyperbolic curve, similar to that already found for 

 nitric acid. — (Ann. chim. phys., Oct., 1882.) c. b. p. 



[191 



Specific heat. — A modified form of Eegnault's 

 apparatus has been employed by M. W. Longuinine 

 for the determination of specific heats. By the revo- 

 lution of the cylinder the body is dropped through 

 the floor of the chamber, in which it is heated, 

 through a space of O.OSm. into the calorimeter. In 

 order to obtain accurate results, it is necessary for the 

 substance to have a spherical form. When powders 

 and similar substances are used, they are placed in a 

 sphere of brass, the specific heat and weight of which 

 are known. This apparatus appears to give more 

 uniform results than Eegnault's. — (Ann. chim. p/'7/s., 

 Nov., 1882.) c. B. P. [192 



Change of chemical constitution by heat. — 

 Herr E. Wiedman has shown that a number of salts 

 containing water undergo chemical change when 

 heated, though the temperature is below that of 

 fusion. He has thus found two new modifications 

 of the sulphates of zinc and magnesium. The result 

 has interesting bearings in the determinations of ten- 

 sion, and of the heat of solution. — ( Wied. ann., No. 

 12.) c. B. P. . [193 



Electricity. 



A determination of the ohm in absolute 

 measure. — Notice was given by A. L. Kimball of 

 the proposed redeternynation of the ohm ; the method 

 to be used being the same as that used by Prof. Row- 

 land in 1876, changes being made in the character 

 and arrangement of the apparatus so as to avoid, so 

 far as possible, the repetition of constant errors. A 

 short account was given of the nature and impoitance 

 of absolute measurement in general, in which the 

 derived units are all based on the fundamental units 

 of length, time, and mass, and derived directly from 

 them. The nature of the unit of electrical resistance 

 was then noticed, and attention called to the fact, 

 that, in the electro-magnetic system of units, the unit 

 of resistance bears to the units of length and time 

 the relation of a velocity. Mention was made of the 

 most noteworthy methods that have been used in 

 determining the value of resistance in absolute meas- 

 ure, attention being called to the manner in which 



