578 LIGHT to communicate their colours to the sunbeams which form a rainbow. Our next glimpse of real progress dates from the llth or 12th century, when ALHAZEN (q.v.) 1 wrote a treatise on optics in Arabic, which for five hundred years or more was a recognized authority on the subject. It was, in many parts, founded on the work of Ptolemy, but with considerable additions and improvements. Alhazen gives an anatomical description of the eye, and points out, fairly enough, how with two eyes we see only one image. But he also points out that we see each object, however small, >ja pencil of diverging rays, not (as the ancients imagined) by a single ray. Alhazen accounts for twilight, and shows how by it to measure the height of the atmosphere. He also gives the now generally received explanation of the curious fact that the sun and moon appear larger when rising or setting than when they are high in the heavens. The farther progress of the subject we need not now trace. From the end of 16th century that progress has been extremely rapid. The dates of the more important steps, and the names of their authors, will be given when we treat of these, in their turn, in the course of the article ; and we will give them the additional interest of being presented, when this can readily be done, in the author s own words. Vision. PRELIMINARY STATEMENTS. Before we commence a more rigorous treatment of the subject, it may be well to make a few preliminary statements as to the nature of vision and the conditions for distinct vision. Properly speaking, these belong to OPTICS (PHYSIOLOGICAL) (q.v-), but it is impossible to treat intelligibly any part of our subject without presupposing some, generally very slight, knowledge of other parts. And the few preliminary state ments we have now to make are in no respect theoretical, while they are so simple that any one may at once test their truth for himself. Distance Except in the case of a very abnormal eye (extremely of most short-sighted or long-sighted as the case may be) there is a distance from it usually somewhere about 10 inches at which if an object be placed it is seen more distinctly than if placed at any other distance. Almost every one, perhaps without knowing it, habitually places at or about that distance from his eye an object which he wishes to examine carefully. When he. places it at a smaller distance he becomes conscious of the effort required to see it distinctly. He has, in fact, to alter the form of the optical machinery of the eye, by a muscular effort, so that it may become capable of bringing to a focus on the retina rays more divergent than those for which the parts were in their unstrained state adapted. A corresponding effort, but usually much more slight, is commonly felt to be required if the object be at a distance greater than 10 inches. Limits of Hence we arrive at the conclusion that, for the minimum distinct o f strain on the eye, rays should fall on it diverging as if they came from a point about 10 inches distant. But for all ordinary eyes any divergence from double of this (i.e., 1 The proper name of this geometer is El-Hasan (or by other accounts Mohammed) ibn el-Hasan ibn el-Haitham, and it is as Ibn el-Haitham that he is commonly referred to. See Woepcke, EMg&n d Omar Alkhayydmi (Paris, 1851), p. 73 sq., and Bar Hebneus, Chron., p. 221 sq. Several of his mathematical treatises exist in English libraries (see the Catalogues of the Br. Mus., Bodl., and India Office MSS.) ; but the only copy of his great optical work the Kit&b el Manazir known to be in Europe is No. 1011 of the Leyden collection, with the commentary Tankih el Manazir of Kemal ed-Dfn Abu l Hasan (Cat. Cod. Or. Lugd. Bat., iii. 6i). A smaller work (Woepcke, ut supra) was based on the optical treatises ascribed to Euclid and Ptolemy, and Ibn el-Haitham claims to have restored the lost first book of the latter. The Arabs had Euclid s Optics (Kitdb el Manazir) in the version of Nasir ed-Din Tusy (S. KhaL, No. 10,532 ; Loth, MSS. of India Office, No. 743). divergence as if from a distance of 5 inches) to zero (i.e., parallel rays) is consistent with the possibility of distinct vision. Rays either more divergent than the former limit, or convergent, are unfit to produce distinct vision. Hence every optical instrument, whatever be the reflexions or refractions to which light has been subjected in passing through it, must finally allow the light to escape either iu parallel rays or with a divergence within the above specified limits, if it is to be employed by an ordinary eye. The comparatively slight differences which exist among ordinary eyes are easily compensated by the rack-work, or screw adjustment, which is invariably attached to the eye-piece of a good telescope and to the body of a good microscope. Every motion of this rack-work alters the divergence of the rays as they finally escape from the instrument. Any eye, however abnormal, if it be capable of producing dis tinct vision at all, has only to be furnished with suitable spectacles in order that it may behave exactly as does a normal eye. This statement, however, refers only to sharpness of definition, not in any degree to colour. The deficiency which causes colour-blindness cannot be supplied by any conceivable process. A definite part of the ordinary organ of vision is wanting (or inactive) in such cases while the merely optical parts of the eye are usually in perfect order. Another fact which must be stated here is that, to pro- Inverted duce vision of a body in its natural position, the image on ""-igc on the retina, as seen from the back, must be inverted not ttl f. rctiin merely as regards up and down, but also as regards right and left. Thus, in the ordinary astronomical telescope, the image on the retina is not inverted, and we therefore see an inverted image. A third is that our judgment of the relative distances of Jr.dg- objects is formed mainly by the use of the two eyes simul- ien t of taneously. One eye, kept still, can inform us only of (llstance - relative distance in virtue of the greater or less effort to see distinctly (already spoken of). With both eyes, or with one eye moved from side to side, parallax comes in, and gives us the stereoscopic effect, as it is called. This power of judging distance is, of course, greater as the eyes are set more widely apart. There is, practically, no limit to the effective distance between the eyes when the proper instrumental methods (as with the telestereoscope) are employed. It is also necessary to premise a few words about colour. Colour. The various homogeneous rays of the solar spectrum have each a colour of its own which no refraction can modify. But what about the many colours which do not occur in the spectrum ? To such a question as " What is yellow " ? the answer is, " Each particular kind of yellow may be any one of an infinite number of different combinations of homo geneous rays." And the same is true, in general, of all other colours. Clerk Maxwell found that a yellow equiva lent to that of the spectrum can be obtained by mixing in proper proportions certain homogeneous red and green rays. This single example is sufficient to show that the colour- sense is of a very singular nature. This question will be fully treated in OPTICS (PHYSIOLOGICAL); but for our present purpose it is only necessary to say that we now know (after Wiinsch and Young) that the normal eye has only three colour-sensations a red, a green, and a violet, and that the apparent colour of any light which falls on it depends merely on the relative intensities of the excitement produced by the light on the three organs of sense corresponding to these sensations. This is true, however, only within certain limits of intensity ; for extremely bright light, whatever be its real colour, seems to excite all the three sensations simultaneously, much as white light does ; and with very feeble light (as, for instance, that of an ordinary aurora or of a lunar rainbow) we are sometimes scarcely conscious of
