352 



PRACTICAL PHYSIOLOGY. 



[LXXIV. 



hole and the right-hand image disappears. Bring the retina posterior to the 

 principal focus, and again there are two images. On closing the right-hand 

 hole the left hand image disappears, and vice vcrsd. 



2. Hering's Apparatus for Mixing the Colours of Coloured Glasses. 



By mixing two primary colours (red, yellow, green, blue), one may obtain 

 all intermediate hues, and by mixing three colours (red, green, and blue, or 

 yellow, green, and violet), one can obtain white. The apparatus consists of a 

 box k (fig. 273), two pieces of mirror glass (* and *,), each placed at an angle of 

 45 to the horizontal plane as shown in fig. 274. The base of the box consists 

 of a coloured glass (/ 2 ), while the lower half of the right lateral wall is 

 filled with the coloured glass plate (/,), and the upper half of the left wall by 

 the coloured glass plate (/). The white gkss plates (W, W,, W 2 ) reflect light 

 through the coloured glasses (fig. 273). The light transmitted from below, 



. 273. Hering's Apparatus for Mixing 

 Coloured Light 



W, W,, W* 



k. Box; t, *]. Lids: 

 White reflecting surfaces. 



FIG. 274. Scheme of Fig. 273. /,/ lt / 2 . Col- 

 oured glasses. W, Wi, JF 2 . White reflect- 

 ing surfaces ; *, *i. Glass plates. 



and that from the two sides, is transmitted by a tube to the observer's eye. 

 The brightness can b3 varied by adjusting the white reflecting surfaces, which 

 are placed opposite a well-lighted window. By means of three small metallic 

 doors (t, , ) any one of the colours can be cut off. Thus any combination of 

 coloured lights can be made, as the glasses are movable. The writer has 

 found it Lest to put the violet or blue lowermost. 



3. Mixing Colour Sensations. 



(a.) Lambert's Method. On a black background place a blue 

 wafer or square of blue paper, and 6 or 7 inches behind it a 

 yellow square or wafer. Hold a plate of clear glass vertically, 

 about 10 inches above and midway between the two squares. 

 Look obliquely through the glass, and get the reflected image of 



