22 Prof. H. W. Dove on a New Photometer. 



With a very uniform dead polish the influence of the material 

 prevails. In this respect a brass disc gave somewhat greater 

 brightness than a copper disc. 



In this case the vertical direction of the instrument is prefer- 

 able, because in rotating while in a horizontal position, the illu- 

 mination of the other side of the photographic picture changes. 

 If the surfaces diffusing light surround a given space, for 

 instance the walls of a room, it is merely necessary, the micro- 

 scope standing upright, to direct the concave mirror to different 

 parts of the room, to show surprising differences by sudden 

 transposition of dark into light. 



Examination of Luminous Bodies. 



In the preceding investigations, the chief point was, how much 

 of the brightness of a given illumination is lost when it is subject 

 to absorption. The want of a unit of light is here less felt than 

 in the case of self-luminous bodies. As nature has not given us 

 this unit, an approximation has been sought in normal candles, 

 the light from which, if the length of the Same is kept the same, 

 is tolerably constant. Other methods are by means of Argand 

 lamps, or gas-flames from given apertures under constant pres- 

 sure, or, finally, by means of a platinum wire which closes a circuit 

 of given intensity. Uncertainty affects all photometric methods 

 to the same extent, if it is required to reduce a given intensity of 

 light to an absolute unit. 



At first it would appear simplest to place the horizontal 

 microscope in such a position that its visual axis shall cut the 

 line joining, the two flames whose intensities are to be compared, 

 in that point at which the photographic image appears. But it 

 is much more convenient to direct the microscope successively 

 upon the two flames, and to alter their distance so that the con- 

 stant light which from a definite distance illuminates the object 

 makes the writing disappear. If both lights are in the prolongation 

 of the visual axis of the microscope, the action of both lights is 

 obtained directly after one another, if behind the first a screen 

 is placed which conceals the second, and then the first and the 

 screen are simultaneously removed: this can be effected so 

 rapidly that the flames, which are considered to be constant, 

 may be assumed to be really so. In flames not so bright, a 

 twentyfold magnifying power may be used. In the case of 

 a gas-flame at a distance of some hundred feet, the passage 

 of black print on dark ground to light upon dark ground 

 is very distinctly seen if a spirit-lamp containing alcohol satu- 

 rated with salt is held in front. The feebleness of the light 

 of some flames as compared with others is surprising. If the 

 instrument is directed upon a stearine candle at a distance of 



