BEYIEW8 — EESEAECHES OK COLOTJE'BLlNDKESS. 157 



and can be unscrewed so as to allow discs of coloured paper, perforated In the 

 centre to receive the spindle, and with a slit corresponding to a radius of the disc, 

 to be placed on the upper surface of the top, the rim or circumference of which is 

 divided into 100 equal parts. The paper discs admit of being placed above each 

 other, and any portion of one disc may be made to appear above another, by pas- 

 sing one edge of its slit through the slit m the other. 



" Thus, let a disc of red and a disc of white paper be placed together on the top, 

 the white being the lower of the two ; we may then, if we choose, cover the white 

 entirely by the red, so that the latter only shall appear ; or at will, bring the 

 white through the slit in the red so as to let one-tenth, one-twentieth, one-twelfth, 

 or the like quantity of the surface of the white cover that amount of the surface 

 of the red. When the top is made to spin, one of the tints (dilutions with white) 

 of red will be obtained, and the quantity of red and white in it may be measured 

 by the graduation on the circumference of the circle. 



" In the same way a circle of red and a circle of black will give the shades 

 (deepening* Avith black) of red; and the delicacy of an eye in distinguishing the 

 nicer gradations of colour, may be quantitatively determined." 



" Again, small discs (half the diameter of the larger ones) of green, and of white 

 or black paper, may be placed on the colour-top above the larger red and white 

 or black discs, so that when the top is spinning, a green circle, surrounded by a 

 red ring, will be visible to a normal eye, and theBe may be compared throughout 

 their tints and shades." 



We cannot help thinking that this beautiful instrument will be to 

 the theory of colour what the thermometer has been to the theory of 

 sensible heat, but before touching on the hypothesis of Mr. Maxwell, 

 we must beg leave to take strong exception to the manner in which 

 Dr. Wilson speaks of the view taken by Sir David Brewster of the 

 constitution of the Solar Spectrum. We can willingly excuse our 

 Author for using (as indeed he was compelled to do for want of 

 better,) the ordinary, vague and indefinite names of colours, such as 

 red, blue, and the like, and also for adopting, as a convenient classifi- 

 cation the distinction of colours into primary or secondary, (a dis- 

 tinction in part certainly arbitrary, probably wholly so,) but when we 

 find him speaking with favour of au hypothesis which asserts, as Sir 

 David's does, that there exist in white light only three homogeneous 

 tints, and that a superposition of three equal lengths of these is the 

 real constitution of the Solar Spectrum as revealed to us by the 

 prism, we would remind him that the experiments on which the 

 illustrious philosopher of St. Andrew's grounds his opinion, have been 

 persistently rejected by names of no small authority ; that the con- 

 clusions drawn from them have been repudiated by a still larger 

 class of savans ; that the hypothesis has found no favour among con- 

 tinental philosophers ; and that the repetition of these experiments 

 by Helmholtz and Bernard has not led to their confirmation ; and 

 further, that some of the facts of colour-blindness brought out in this 



