268 Dr. S. Bidwell. On Negative After-images, and 



slit-frame) that even when the slit-frames are in contact with one 

 another, and the slits are opened to their widest extent, no obstruction 

 to the passage of the light through the slits is presented by the rollers. 

 Each slit-frame can be moved independently to any desired position, 

 and clamped with a set-screw. On the other side of the slit-screen a 

 second pair of guides is fixed, each having three parallel saw-cut 

 grooves in it. These guides carry rectangular pieces of sheet zinc of 

 various widths, which may be used to shield temporarily one or more 

 of the slits when it is desirable that its adjustment shall not be dis- 

 turbed. In some experiments it is necessary to use larger portions of 

 the spectrum than can be transmitted by the slits ; the slit-frames are 

 then removed from the screen, and the spectrum dealt with solely by 

 means of the zinc plates. Pieces of zinc sliding in different pairs of 

 grooves may be made to overlap one another, thus providing screens 

 or openings of almost any desired width with very little trouble. 



The diameter of the lens generally used at K, fig. 3, is 10*2 cm. 

 (4 inches), and its focal length 305 cm. (12 inches), the diameter of 

 the circular colour-patch projected upon the screen being then only 

 about 1-5 cm. This size was, however, amply sufficient for most pur- 

 poses, and with a larger image the necessary luminosity could not 

 always be obtained. Sometimes a lens having a focal length of 

 40-6 cm. (16 inches) was used at K, the diameter of the patch then 

 being 2 cm. 



The focal length of the lens M is 12*7 cm. (5 inches) ; it is sur- 

 rounded by a broad diaphragm to screen off stray light. is a device 

 known to photographers as a "rotating diaphragm"; it has eight 

 apertures ranging from 0*21 cm. to 1'42 cm. in diameter, any one of 

 which can be placed in the path of the beam of light. Its object is to 

 vary the luminosity of the white-light disc projected upon the screen L. 

 The lens Q has a diameter of 6*5 cm. and a focal length of 16 '5 cm. 

 (6J inches). 



Wave-lengths of the Colour-patch Light. — No attempt was made to 

 standardise the spectrum projected upon the slit-screen, the wave- 

 lengths of the light illuminating the colour-patch being determined, 

 when necessary, by means of the spectroscope E, fig. 3. The opaque 

 white screen L being removed, a screen of ground-glass is put in its 

 place, and the slit of the spectroscope is brought near the bright image 

 on the glass. The purpose served by the ground-glass is to diffuse the 

 light, so that any element of the light transmitted by the slit-screen 

 may be at once examined without the need of turning the spectro- 

 scope in its direction. The spectroscope has a six-inch circle with a 

 vernier reading to minutes ; the prism is of extra dense Jena glass, the 

 refractive index for D being 1'693. To ascertain the constitution of a 

 colour-patch, the deviations corresponding to the two extremes of the 

 one or more coloured bands seen in the spectroscope are determined, 



