COLOR-BLINDNESS of yellow and light blue; while indigo and Prussian blue he pronounced black. Dr. Dai- ton could not by daylight tell blue from pink ; he scarcely saw the red of the spectrum, and considered the remainder of it as showing but two colors. But a failure to perceive the more refrangible rays is most common. Seebach con- cludes that all eyes, however imperfect other- wise, see yellow ; and that the sensations of complementary colors are inseparable, so that if the eye be sensible or insensible to either it will be so to both, the eye that fails to see orange also mistaking blue, &c. In the third degree, however, admitted by other ob- servers, all colors are recognized only as giv- ing certain degrees of light or shade. This form is rare. Dr. Wilson found but one case ; and in this some of the colors could be named by gas light or transmitted light, none by reflected daylight. The color-blind very often do not know their own defect; and in the lower walks of life their lack incapacitates them for cer- tain employments, or may even imperil life. These evils particularly befall weavers, tailors, gardeners, railway attendants, sailors in the steam service, and others dependent on the use of colored articles or the perception of colored signals. The importance of a correct percep- tion of colors, in the present modes of signal- ling upon railways and shipping, cannot be overestimated. For example, the English ad- miralty orders require a,t night a green light on the starboard, a red light on the port side of vessels ; and by the color the steersman must know which -side of the vessel is toward him, consequently whether it is going to right or left, and whether to starboard or port his helm. Although no case of accident has yet been traced to color-blindness in the attendants, yet such a result is easily conceivable, especially as their powers of vision are not tested; and the most doubtful complementaries, red and green, are much in use as signals. Practical inferences are that the ability in this respect of candidates for the posts of sailors and rail- way men should always be first carefully test- ed ; but, better still, that form and position of signals should, as far as practicable, be sub- stituted for color, as the former are qualities less liable to be mistaken, and the color-blind generally perceive form even more correctly than other persons. The cause of color-blind- ness probably lies somewhere between the eye, as an organ, and the mind ; or more correctly, in a want, partial or total, of a certain percep- tive faculty, that of color, as an element of ac- tive mind. Dalton thought the retina or hu- mors of his eyes must be colored, and probably blue ; a nice post-mortem dissection of his eyes revealed no abnormal coloration or appear- ance whatever. Dr. Trinchinetti proposed as a cure the extraction of the crystalline lens ; but Wilson gives a case of cataract, in which color-blindness supervened on the extraction of the lens. In one instance, the latter found the difficulty to follow permanently on concus- OOLORIMETER 109 sion of the brain ; sometimes it was tempo- rary, and dependent on congestion, dyspepsia, or hepatic derangement; most frequently it was congenital. Color-blindness is generally hereditary. Leber, who examined many cases, found it a frequent sympton of atrophy of the optic nerve, and of scotoma (muscce volitante*, &c.). Dr. Argyll Robertson (Edinburgh " Medi- cal Journal," February, 1869) found it accom- panying a case of spinal disease. Dr. Chisholm of Charleston, S. C., observed it in a case of in- flammation of the optic nerve. It is often, how- ever, unaccompanied with any impairment of vision. It has been observed during pregnancy, and Lawson met with a case which was pro- duced by over use of the eyes in sorting colors. As has already been noticed, the ethereal waves of light in the different colored rays vibrate in different times, the number of vibrations in the middle red ray being about 477,000,000,- 000,000, while the number in violet light is 699,000,000,000,000 times in a second. There are also waves on either side of these limits which are too slow on the one hand, and too rapid on the other, to be perceived by the hu- man eye, just as some vibrations in the air may be too slow or too rapid to be perceived by the ear. That some persons can perceive a lower tone of red, or the more extreme rays of the violet spectrum, as well as that some can perceive lower or higher notes in music, is a matter of observation, as also the fact that the perception of the depth and tone of various colors varies in different individuals. It is then a matter of no great surprise that in some persons the retina should fail to perceive the difference between the vibrations which take place in some of the colored rays. In the con- genital cases, and in some others, the attempt at cure by medicines has been found utterly hopeless ; of a cure through education no case is established. Th,e want may be alleviated by carrying about a chromatic scale, named, for purposes of comparison ; but little help is thus derived. It is strange that the substitution of artificial for solar light seems as yet to otter decided relief in the largest number of cases ; and a draper has been known to keep his shop lighted with gas during the day for this pur- pose, and with success. Very white light is less useful in these cases ; the best being a light yellow by passing through glass stained with preparations of silver, uranium, or iron. Dr. Wilson found that a good test for persons con- founding red and green, and who may be un- aware of the fact, was obtained by placing be- fore their eyes a red glass; the beholder is at once astonished at the difference which he dis- covers in looking at the two colors. COLORIMETER, an instrument for measuring the depth or color in a liquid by comparison with a standard liquid of the same tint. The comparison is made either by varying tho depth of the stratum of liquid under examina- tion till it exhibits the same intensity of color as the normal liquid, and then measuring the
