6 PROCEEDINGS OF THE AMERICAN ACADEMY. 



(Phil. Trans., 199, B, p. 250) which appears to be a good example. 

 There seems to be no intrinsic reason why plus variations from the 

 normal should be less frequent than the minus variations, and they 

 probably are not so in fact, the apparent reason being the wrong diag- 

 nosis likely to be made with the ordinary method of testing. Thus (1) 

 and (5) are quite certain to be confused in the worsted tests most 

 commonly employed, likewise (2) and (4) while plus or minus varia- 

 tions in the blue unless very marked would almost certainly escape 

 detection. 



The spectroscope and the color mixing apparatus give the only 

 reliable methods of testing for minor variations, and the writer has used 

 in his own experiments a simple form of spectroscopic test which may 

 be worth describing here. The chief test is in principle the converse 

 of Rayleigh's, and consists in matching a synthetic yellow and a syn- 

 thetic blue-green, of the same hues as the spectral colors corresponding 

 to the red-green and blue-green junctions for the normal eye, by 

 shifting a pure spectrum which occupies the lower half of a slit in the 

 focal plane of the eyepiece while the synthetic color occupies the upper 

 half. 



The apparatus is shown in diagram in Figure 2. The basis is a simple 

 constant deviation spectroscope mounted in a capacious wooden box 

 to avoid stray light. The prism P is mounted as usual on a turntable 

 b, rotated by the milled head D with a screw bearing on a steel plane 

 on the turntable. On the screw shaft is a wide pinion /, engaging a 

 narrow rack ending in a pointer g, moving over the scale h. The 

 rack works under the turntable freely through guides. The main slit, 

 S, has the usual collimating lens c, of 4 cm. diameter and 15 cm. focus. 

 The observing telescope 0, of about 4 cm. diameter and 35 cm. focus 

 has a compound eyepiece slit e, a slider with an adjustable slit and a 

 clear aperture for viewing the spectrum. Long screws k, h, position 

 the slider so that when slipped to the right the slit can be brought any- 

 where in the spectrum, and when slipped to the left the field is wholly 

 or partly clear. 



Beyond the turntable is a second collimator with adjustable slit S' 

 and lens c' 12 mm. in diameter. This ranges over the prism P which 

 is 25 mm. thick and the beam from c' is turned into the upper part of 

 by the mirror m. A small 60 degree prism can also be placed in 

 front of c' to furnish a reference spectrum. Condensing lenses L, V 

 direct the light from sources /, V upon the slits. 



In using the apparatus the filter i is placed in front of S'. For 

 synthetic yellow this is made of opposed wedges of cobalt and selenium 



