ON ACCIDENTAL OR SUBJECTIVE COLORS. 237 



short focus, irradiation became too small to he perceivefl It is evident from 

 this that in astronomic observations the eye-glass of the telescope must exert a 

 decided action upon the irradiation which surrounds the image of a star. 



M. Plateau has shown that the modification which irradiation undergoes from 

 the interposition of a lens seems to be controlled by the following laws: First, 

 irradiation is diminished by convergent lenses; this effect, considerable when 

 the focal distance is short, grows weaker as that distance increases, be- 

 comes null when the latter is infinite, and changes its sign with it ; that is to 

 say, irradiation increases, on the contrary, imder the influence of divergent 

 lenses Secondly, experiments seem to indicate that the action of the lenses 

 seems to depend only on their focal distance, and not on the absolute curvature 

 of their surface. 



M. Plateau concludes his admirable memoir, written with the utmost clear- 

 ness both of reasoning and style, by presenting, under the form of a summary, 

 the following conclusions drawn from the doctrine of irradiation: 



I. Ocular irradiation. — 1. Irradiation is a well-established fact, easy to 

 verify, very variable, but capable of being measured with precision under all 

 circumstances. 2. It is manifested at all distances from the object which pro- 

 duces it, from the shortest distance of distinct vision to any degree of remote- 

 ness whatever. 3. The visual angle which subtends and which measures it is 

 independent of the distance of the object. 4. It follows from this that the 

 absolute breadth which we attribute to it is, all else being equal, proportional to 

 the distance which exists, or which appears to us to exist, between the object 

 and our eyes. 5. Irradiation increases with the brightness of the object, but 

 according to a law much less rapid. If this law be represented by a curve 

 having for abscissfe the successive values of the brightness commencing at 

 zero, and for ordinates the corresponding values of the irradiation, rhis curve 

 passes by the origin of the co-ordinates, turns its concavity towards the axis of 

 the abscissae, and presents an asymptote parallel to that axis. The curve is 

 already very near its asymptote for a brightness of the order of that of the 

 northern sky. 6. When the field which surrounds the object is not completely 

 deprived of light, the irradiation of that object is diminished, and the more 

 strongly in proportion as the brightness of the field approximates to an equality 

 with that of the object. If equality takes place, irradiation vanishes. 7. Henco 

 it follows that when two objects of equal brightness adjoin one another, 

 irradiation is null for each of them at the point or line of contact. 8. Two 

 irradiations in presence of and sufficiently near one another, both suffer diminu- 

 tion This diminution is the more considerable as the borders of the luminous 

 spaces whence the two irradiations emanate are in closer proximity. 9. Irra- 

 diation increases with the prolonged contemplation of the object 10. In the 

 same individual and for an object of the same brightness, the irradiation varies 

 considerably from one day to another. 11. Irradiation is modified when a lens 

 is placed before the eye; it is diminished by convergent lenses, and augmented 

 by divergent lenses. 12. This action of lenses seems to depend only on their 

 focal distance, and not on the absolute curves of their surfaces. It appears the 

 more decided in proportion as the focal distance is shorter. 1.3. The most 

 probable cause of irradiation seems to be the one now generally admitted, 

 namely, that the excitation produced by the light is propagated on the retina a 

 little beyond the contotir of the image. By means of this principle, which is 

 besides sustained by facts, we aie able to give a reason for ali the laws of irra- 

 diation observed with the naked eye; but difficulties are encountered when the 

 action is exerted by lenses. 



II. Irradiatiun obsernfd through astronomical instruments. — 14. The error 

 produced in astronomical observations by wh.(t has been called in that case 

 irradiation, proceeds from two causes essentially distinct — ocular irradiation and 

 the aberrations of the telescope. 15. The part in that error which is due tc 



