668 ON THE RELATION OF NATURAL SCIENCE TO ART. 
other things, he explained the relation between the different intensities 
of light in objects of the actual world and those on the painter’s pal- 
ette, and pointed out the means by which the difficulties arising there- 
from may be overcome.* Thus painters, as von Briicke remarks, have 
it in their power to reproduce the dazzling effect of the disk of the sun 
by imitating the irradiatio al perception, the true 
nature of which was recognized by von Helmholtz. An example of 
this, interesting through its boldness, is the lovely Castell Gandolfo in 
the Raezynski gallery 
There are so many and striking instances of such imperfections of 
the human eye that, notwithstanding its marvellous capabilities, von 
Helmholtz has observed that “he would feel himself justified in cen- 
suring most severely the careless workmanship of an optician who 
offered him for sale an instrument with similar defects, and that he 
would emphatically refuse to take it.” The eye being the chief organ of 
artists, its defects are of great importance in art and its history, and 
artists would do well to inform themselves not only on these defects in 
general, but more particularly on those which they, in their own per- 
Sons, are subject to; for, as Bessel remarked of astronomical instru- 
ments, ‘an error once well ascertained ceases to be an error.” 
Our conception of the stars as stars, in the shape adopted symboli- 
cally by decorative art, is caused by a defect of the eye closely related 
to irradiation, stars being luminous spots in the sky without rays, as 
they actually appear to a privileged few. Prof. Exner, whose line of 
thought we shall repeatedly cross in the course of these reflections, 
justly remarks that to this imperfection the stars conferred by sover- 
eigns as marks of distinction owe their origin and starfishes their name 
even since Pliny’s time. The different varieties of halo, however, are 
more probably freeborn children of our fancy—from the Byzantine 
massive golden disk down to the mild phosphorescence proceeding from 
holy heads and in Correggio’s “Night” from the entire child, which 
illumines the scene with a light of its own. According to Prof. Exner, 
glories of the latter description are derived from the radiance which 
surrounds the shadow of one’s own head in the sunshine on a dewy 
meadow, and which in fact has always been compared to halos in reli- 
gious pictures. This phenomenon even misled Benvenuto Cellini into 
the pious delusion that it was a gift granted him individually from 
above, and a reflection of his visions, such as Moses brought down from 
Mount Sinai.t 
Certain otherwise quite inexplicable peculiarities which disfigure the 
later works of the distinguished landscape painter Turner have also 
been traced to defects of the eye by Dr. Richard Liebreich.¢ Clouded 
* Prof. von Honolte SEE Eee and ieee i iD Bente ick, 1884. 
t‘* Vita di Benvenuto Cellini, seritta da lui medesimo,” libro primo, CxXxv1l. 
tTurner and Mulready: The Kffect of certain Faults of Vision on Painting, &c. 
London, 18838. 
