238 ON ACCIDENTAL OR SUBJECTIVE COLORS. 



ocular irradiation depends on the enlargement itself, on the brilliancy of the 

 image, and on the eye of the observer. It is, moreover, considerably diminished 

 by the action exercised by the eye-glass of the telescope, as a convergent lens 

 placed before the eye, and this diminution is greater as the eye-glass is more 

 powerful. As regards the eye of the observer, the effect will differ from one 

 person to another, and, for the same person, will vary from one period to another. 

 16. This same part of the total error vanishes in observations in which a 

 micrometer a dovble image is employed. 17. The other part of the total error 

 which proceeds from the aberrations of the telescope varies, of course, with 

 different instruments; but for the same telescope it may be considered as con- 

 stant 18. The effect of ii-radiation in telescopes, or the total error proceeding 

 both from the ocular irradiation and the aberration.'? of the instrument, is neces- 

 sarily variable, since it depends on variable elements ; it may be insensible iu 

 certain cases, and attain a considerable value in others. 19. It is possible, even 

 with an indifferent telescope and an eye very sensitive to irradiation, to obtain, 

 by help of certain processes, results which may be considered as disengaged 

 from this total error. 



Objections of M. Arago and reply of M. Plateau. — On presenting to the 

 Academy of Sciences the able memoir which we have been analyzing, M. Arago 

 developed various considerations which led him to dissent from the physical 

 explanation which the accomplished professor of Ghent has given of his experi- 

 ments. M. Arago also recurred to the observations made twenty-five years 

 before by himself in order to ascertain whether the measurements of the planetary 

 diameters, taken with his telescope a douhle image, were affected by any irra- 

 diation ; he spoke of the influence which it would seem that diaphragms placed 

 before the object-glass might exert over measurements of this kind, since they 

 augment so sensibly the diameter of the stars. He announced, in fine, the 

 early publication of a memoir in which he would take occasion minutely to 

 record the result of researches which, on the one hand, touch upon physiology, 

 and, on the other, connect themselves with several of the most important ques- 

 tions of astronomical science. This promise was made in 1839 ; but the memoir 

 never appeared. The following is the decisive reply of M. Plateau, as entered 

 in the proceedings, to the objections of the distinguished secretary of the 

 Academy : 



" In the session of the 6th of May last M. Arago occupied the attention of 

 the Academy with my memoir on irradiation, and presented at the same time 

 some observations on the theoretical part of the work. He thought that the 

 physiological explanation which 1 had sought to establish could not be main- 

 tained, and advanced a new theory, according to which irradiation is the result 

 of the chromatic aberration of the eye. M. Arago's reasonings not having been 

 printed, I am but imperfectly informed of them, and do not know how far they 

 tend to refute the arguments which I have adduced in favor of the physiological 

 theory. I shall therefore not repeat those arguments, but shall confine myself 

 to an examination of the new hypothesis presented by M. Arago. 



" It is true that physicists do not now recognize the eye as a perfectly achromatic 

 instrument, and from this non-achromatism it necessarily follows that the images 

 of objects on the retina are surrounded with a small band of aberration, which 

 must slightly augment the apparent dimensions of luminous objects projected on 

 a dark ground, and diminish that of dark objects projected on a luminous 

 ground. But is this effect perceptible under ordinary circumstances, and has 

 the small band of aberration sufficient breadth to allow us to distinguish it, and 

 to attribute to it the known phenomenon of irradiation 1 This is the question, 

 and one, 1 thmk, capable of solution. 



" Let it fiist be remarki-d that by virtue of the very cause which produces it 

 the small band which the chromatic aberration of the eye generates around 

 images cannot be devoid of colors. Consequently, if the irradiation manifested 



