PHYSICS. 481 



property which it possesses of rendering distinguishable upon a white 

 screen ilhiniinated with it minute details, as lines, etc. If, now, the light 

 be weakened till they vanish, the ratio of the initial intensity to this 

 limit of intensity will be sensibly constant and is a function only of the 

 wave-length used. The light to be tested is therefore received normallj' 

 on the slit of a spectrophotometer covered with a strip of glass, on which 

 is photographed a series ot line and close dividing lines, their direction 

 being perpendicular to the direction of the slit. A pure spectrum is 

 seen furrowed by fine longitudinal striae. If, now, the eye-piece slit \ye 

 made to include any particular region, and the nicol prism be rotatetl 

 till the strife cease to be perceptible, a method is secured for compar- 

 ing intensities. {Compes Rendus, xciii, p. 959; J. Phys., April, II, i, p. 

 162; Phil. Mag., January, V, xiii, p. 72.) 



Briicke, finding that objects cease to be visible at a greater visual 

 angle the more they differ in color but not in brightness from the 

 background on which they are seen, has made use of this principle in 

 photometry, A board, black at one end and white at the other, be calls 

 a brightness table. It has successive shades of gray between. To de- 

 termine the brightness of a colored paper, for example, he places a bit 

 of it before different parts of the board and notes the place where with 

 the shortest interval it becomes invisible. {Nature, June, xxvi, p. 138.) 



Koenig has reported to the Physical Society of Berlin on a new in- 

 strument designed and constructed by Helmholtz, which he calls the 

 leukoscope. It consists essentially of a calc-spar rhomboid, a plate of 

 quartz, and a nicol prism. When a luminous pencil enters the calc-spar 

 it is split into two rays polarized in planes at right angles, which trav- 

 erse the quartz plate and the nicol. When analyzed by a prism these 

 rays give two spectra containing absorption bands, those in the one 

 being the complement of those in the other. The instrument is used by 

 l)utting in a quartz plate of the proper thickness and then rotating the 

 nicol until white light is produced. When different sources of light 

 are employed, different degrees of rotation are necessarj- to effect this 

 result, the relative quantity of different colors in different lights being 

 different. The angle of rotation for stearin candles was 7I°.20; for gas 

 light, 71^.5; for electric arc, 7d^; magnesium light, SOO; solar light 90o.o. 

 The calcium light and burning phosphorus gave angles between gas and 

 the electric light. Experiments with the Edison and the Swan incan- 

 descent lamps showed that the luminosity increases much more rapidly 

 than strength of current, the illuminating power being increased sixty- 

 fold by doubling the current. The angles of the leukoscope increase as 

 the light rises; so that a curve plotted with light intensities as ab- 

 scissas and angles as ordinates is concave towards the axis of abscissas, 

 approaching asymptotically a maximum near 78° ; an angle approxi- 

 mately equal to the angle of the arc. {Nature, November, xxvii, p. 9o.) 

 H. Mis. 26 31 



