502 Mr. E. S. Johonnott on the Thickness of 



Two . methods, both suggested by Prof. Micbelson, were 

 used in making the measurements here given, 

 the first, an interferometer method, 

 the second, a photometric method. 



In the first method continuous readings, for the mean thick- 

 ness of a number of films, were made over a period of several 

 hours. In the second method the thickness of a single film 

 was determined from the intensity of the light reflected at a 

 series of different angles of incidence. Since it was impossible 

 to obtain monochromatic lights sufficiently intense, this method 

 does not furnish accurate results. 



The liquid used was a solution of either oleate of soda or 

 oleate of potash in distilled water, the proportions ranging 

 from 1 part of oleate to 100 of water (1 : 100), to 1 part of 

 oleate to 40 of water (1 : 40). Tests were made with potassium 

 nitrate and also glycerine added to the above solutions. 



The index of refraction of all the solutions was assumed to 

 be that of pure water, viz., 1*333. All the measurements 

 were made on vertical films, formed on rectangular frames 

 made of glass fibre. The lower edge of the films was on the 

 surface of the solution in all cases. 



Method I. 



Prof. Michelson's interferometer was used in making the 

 measurements by this method. 



Description of Apparatus. — The two reflectors were placed 

 at A and B, on a stone slab 2 feet by 2 feet (fig. 1), and the 

 separating surface and compensator at C. The films were 

 brought into the path, BC, of the interferometer, at F, on 

 frames made of glass fibre 0*5 millim. in diameter. In 

 mounting the frames, a ^-inch round brass rod 35 centim. 

 long was slit lengthwise into halves. 60 grooves, at intervals 

 of 5 millim. were ruled on the flat surface of one of the halves. 

 A glass fibre 7 centim. long was clamped centrally in each 

 groove by means of a long square rod faced with chamois 

 skin. Upon levelling the plane of the fibres and carrying a 

 fine flame along the edges of the grooved surface, all of the 

 frames were formed in vertical planes. The frames were then 

 all brought to a common length by forming beads on the 

 lower ends with the flame. These ends were then lowered 

 into molten solder in a trough made from a piece of J-inch 

 brass tubing, slitted longitudinally. After cooling, the clamp 

 was removed, leaving 54 frames rigidly mounted and per- 

 fectly in line at the tops and sides. Each end of the supports 

 of the frames was then attached, by means of small wind- 



