172 Mr. A. M. Mayer on a Photometer for Measuring 



colours of the outer half (a in fig. 17) of the ring formed of 

 three thicknesses of paper, with the same central coloured 

 disks, used in the previous experiment, when one side of the 

 disk was illuminated by lamp-light, the other by daylight. 

 But on increasing the saturation of the hues of the central 

 disks and adding peripheral rings of the same hues, I suc- 

 ceeded in making the hues the same on both sides of the 

 photometer-ring. If equality of hue can be obtained when 

 the photometer is illuminated, on one side by lamp-light and 

 on the other by daylight, then the contrast-colours may be 

 brought to the same hue when the photometer is illuminated 

 on one side by a candle or petroleum-flame and on the other 

 side by the electric-arc light or by the whitest Welsbach 

 incandescent lamp. 



With a change of distance of the petroleum-flame from the 

 photometer, the differences in the illumination of the portions 

 a and b, fig. 17, of the ring were changed. When the two 

 sides of the ring were equally illuminated the rings a and b 

 appeared indistinguishable, fusing into one ring of uniform 

 tint and illumination, of the breadth of a + b. 



With the best Bunsen-photometer disk* I was unable to 

 decide where it received equal illumination on its sides, so 

 difficult was it to judge of equal brightness of the blue and 

 orange on one side when compared with the orange and blue 

 in the same respective positions on the other side of the disk. 



With a petroleum-flame of 40 candle-light-giving power 

 on one side of the rotating photometer and a candle on the 

 other, the delicacy of the indications of the rotating photo- 

 meter equalled, and even slightly excelled, those of the very 

 best Bunsen-photometer disk. 



If we bring in succession the translucent sectors of the 

 rotating photometer-disk between two lights we observe that, 

 on certain sectors, a and b appear as one surface of the same 

 uniform tint and illumination ; on other sectors sometimes a, 

 sometimes b, is the brighter. Rotation of the disk entirely 

 destroys such slight differences, and the disk then acts as if 

 made of absolutely homogeneous material, placed in the same 

 conditions of contact of the layers of paper, in each translu- 

 cent sector. 



The Hue of the Light of " a White Incandescent Welsbach 

 Lamp " compared with Daylight. — When the screen (fig. 1) 

 is illuminated on one side by the Welsbach lamp and on the 

 other by daylight, the side of the ring of the screen facing 



* The best Bunsen photometer-disks I have used are those sold by the 

 American Meter Company of N. Y. They are made in England. 



