day: pigment-migration in eye of crayfish. 311 



By regulating the width of the slot in each diaphragm the radiant 

 energy in the various regions of the spectrinn could be reduced {c. g. 

 in red) or enhanced (e. g. in blue-violet) until an equality, as deter- 

 mined by the radiomicrometer, was established between them. Be- 

 sides the coat of dead black paint on the interior of the box, three 

 diaphragms, two permanent ones of wood at m and r, Fig. A, and a 

 removable one of sheet-iron at s, reduced internal reflections from the 

 walls to a minimum. A small window, w, on one side and near the 

 rear, together with an aperture cut in the lid and surmounted by a 

 metal chimney, served as ventilators to reduce the heat from the lamp. 

 Metal flanges in the chimney and at the side aperture were so over- 

 lapped as to prevent any appreciable leakage of light. By means of a 

 metal peg projecting upward from the table and inserted in a hole in 

 the floor of the box at a point directly beneath the center of the prism, 

 the apparatus was so pivoted that the beam of monochromatic light 

 could be directed at will. 



Illumination was furnished by three 220-volt Nernst filaments of 

 the pattern used in the Schwann lamp and so arranged that they 

 might be used either in combination of three, for blue- violet and green, 

 two, for yellow, or singly, for red light. The feebleness in radiant 

 energy of the blue end of the spectrum necessitated not only a wide 

 slot in the blue diaphragm but, in addition, this reenforcement by 

 triple combination of glowers to obtain sufficient energy to equal 

 that furnished by the single glower and a narrow diaphragm-slot 

 in the red. A front perspective view of the lamp devised for facili- 

 tating these combinations is shown in Fig. C. At one end of a wooden 

 base, a, was erected an arch of brass, b, about six inches high. The 

 three L-shaped posts, p, p', p" , and the arms, r, r', r" , above were 

 also of brass. The middle arm, r, and post, p, were screwed firmly 

 to the underside of the brass arch and to the wooden base, respectively. 

 On the right side of this central combination, rfp, the arm r' was 

 fastened to the arch with a thumb-screw and nut so placed that 

 the pivoting point m plumbed exactly with the screw n in the post 

 p' below. The wooden column h, sheathed with blackened asbestos, 

 connecting the backward extensions of r' and p' , so firmly united 

 these two parts that they could be rotated coordinately about mn 

 as an axis. Whereas the upper platinum connection of the glower /' 

 was wedged into a hole in the arm r' with a copper plug, e, the lower 

 connection, in order to allow for the expansion of the filament when 

 heated, was suspended freely in a mercury-filled cup, c. This brass 

 cup was bound with copper wire to the post p'. The whole system 



