336 H. M. JOHNSON 



glass is placed close behind the test-object, the latter being screwed 

 into the end of the photometer box before a window of suitable 

 size. The brightnesses of the two test-fields are made respectively- 

 equal, each being balanced against a standard lamp. The ap- 

 paratus is so arranged that the test objects are brought directly 

 before, and close to, the corresponding windows of the Yerkes 

 box. These windows are further limited by acid-blackened 

 brass plates, in each of which a circular window 6 cm. in diameter 

 has been accurately cut and beveled, and carefully centered 

 with respect to the test-object and the ends of the alley to which 

 it belongs. 



To present test-bands of a given size, stop St 1 is set in the 

 desired position by the micrometer screw. If stop St 2 be set at 

 a considerable distance (15 mm. for the present gratings) from 

 St 1 , the experimenter can cause the striae to disappear by moving 

 the yoke Y against St 2 , since this increases the angle of rotation 

 enough to reduce the striae to the order of the ruled lines on the 

 gratings. This change can be made from the experimenter's 

 chair, by merely pulling a string attached to the lever L. When 

 the string is released the spring Sp 1 brings the gratings to their 

 former position. If one desires to shift from one size of visible 

 striae to another, as for work in Problem 2 suggested above, the 

 setting of St 2 is of course changed so as to limit the movement of 

 Y to the desired degree in that direction. 



The above procedure has been used successfully in problems 

 1, 2 and 3, with quite satisfactory results. The writer has one 

 suggestion to make as a result of his experience: the grating- 

 width 1-240 inch is satisfactory for work with the human subject, 

 and with animals whose visual acuity is 50% poorer. It can be 

 used at ordinary reading distance, if necessary, but for animals 

 should be placed considerably farther from the eye. For animals 

 whose vision is relatively poor the grating is too fine. ' It is im- 

 possible to present striae whose width is greater than 4 mm. 

 without inconvenience where the distance from eye to test field 

 is less than a meter. The angle of rotation at this reading is so 

 small that disturbance from parallax results at short distances. 

 Further where the angle of rotation is very small, very slight 

 changes in the setting of St 1 cause changes in the width of the 

 test-bands which are inconveniently large. In work on the chick 

 in Problem 2, the range of widths of the test-bands is not long 



