386 BULLETIN OF THE BUREAU OF FISHERIES. 



Since the only photometer available to the author at that time was one intended 

 for use in the air, this form was employed, being adapted as follows : All exposures were 

 made with the photometer placed in a glass preserve jar of sufficient diameter to permit 

 the photometer to lie flat on the bottom. The photometer was held in place by paper 

 packed into the jar, care being taken that the slot for exposing the solio paper was not 

 shaded by the packing. In a dim room the slot was opened, the photometer was placed 

 in the jar and securely packed, and the jar was tightly wrapped in black cloth. This 

 was then taken to the desired situation, the jar being held horizontally with the slot 

 directly on top, the cloth was quickly removed for the desired number of seconds and 

 then quickly replaced, and the jar was then brought back into the laboratory. All 

 changes in the apparatus were made in a dim room at a considerable distance from any 

 window. The standards were obtained in this way by exposing the photometer within 

 the glass jar to direct sunlight on an upper, unshaded, southern porch. 



For exposing the photometer below the surface of the water a shallow box, open at 

 the top and of the proper size to hold the jar horizontally, was built, the sides of the box 

 being just high enough to hold the jar in place and not shading the upper part of it. 

 This box was then fastened to a handle marked with the desired distances. In a dim 

 room the jar containing the opened photometer was placed horizontally in this box with 

 the slot directly on top, and the box w r as tightly wrapped with black cloth. This was 

 carried in a boat to the desired locality and held at arm's length below the w r ater, the black 

 cloth was then removed, and the jar immediately sunk to the desired depth and held at 

 that level for a definite time. The jar was then quickly brought within reach and imme- 

 diately covered with the black cloth, not more than a second being required for this 

 manipulation. The apparatus was then carried to the laboratory, where all changes 

 of the photometer were made in a dim room. 



While the jar undoubtedly diminished the light reaching the photometer, this 

 decrease would be the same in the standards and the tests. The effects of the light in 

 these two cases may, therefore, be directly compared. 



Two records, one at high and one at low tide, were obtained in this way in the 

 channel in front of the laboratory wharf in July, 1907. In the first of these the 

 standards were made from i : 15 p. m. to 2 p. m., July 17, and the measurements 

 below the water were made from i : 15 p. m. to4p. m., July 18, high tide on this day 

 occurring at 2 p. m. In some cases, where the color of the test did not exactly match 

 that of any standard, the time of the standard having an effect equivalent to that of 

 the test was obtained by interpolating between the two standards showing the colors 

 nearest to that of the test. Standards were made by exposing to direct sunlight as 

 described above for 60, 30, 25, 22, 20, 15, 10, 5, 3, 2, and i second. The results were 

 as follows, the first column giving the depth below the surface at which the test was 

 exposed, the second column giving the time of the exposure of the test, the third 

 column giving the time of exposure of the standard having a color equivalent to that 



