Mar. 2J, 1872] 



NATURE 



405 



3. The over-sighted (hypermetropic) eye, on the con- 

 trary, has too weak a refraction ; it unites convergent rays 

 of light upon the retina ; parallel or divergent rays of 

 light it unites behind the retina, unless an effort of ac- 

 commodation is made. (Fig. 3,) 



Hypermetropy, the lecturer explained, does not es- 

 sentially influence painting, and is easily corrected by 

 convex glasses. Short-sightedness, on the contrary, 

 generally influences the choice of subject as well as its 

 manner of execution. 



Sometimes the shape of the eye diverges from its nor- 

 mal spherical form, and this is called astigmatism. This 

 has only been closely investigated since Aii-y discovered 

 it in his own eye. Figure to yourself meridians drawn on 

 the eye as on a globe, so that one pole is placed in front ; 

 then you can define astigmatism as a difference in the 

 curvature of two meridians, which may, for instance, 

 stand perpendicularly upon each other ; the consequence 

 of which is a difference in the power of refraction 

 of the eye in the direction of the two meridians. 

 An eye may, for instance, have a normal refraction 

 in its horizontal meridian, and be short-sighted in 

 its vertical meridian. Small differences of this kind 

 are found in almost every eye, but are not perceived. 

 Higher degrees of astigmatism, which decidedly disturb 

 vision, are, however, not uncommon, and are therefore 

 also found among painters. 



I observed a very curious influence of astigmatism 

 upon the works of a portrait painter. He was held in 

 high esteem in Paris, on account of his excellent grasp 

 of character and intellectual individuality. His admirers 

 considered even the material resemblance of his portraits 

 as perfect ; most people, however, thought he had inten- 

 tionally neglected the material likeness by rendering in 

 an indistinct and vague manner the details of the features 

 and the forms. A careful analysis of the picture shows 

 that this indistinctness was not at all intentional, but 

 simply the consequence of astigmatism. Within the last 

 few years the portraits of this painter have become con- 

 siderably worse, because the former indistinctness has 

 grown into positively false proportions. The neck and 

 oval of the face appear in all his portraits considerably 

 elongated, and all details are in the same manner dis- 

 torted. What is the cause of this? Has the degree of 

 his astigmatism increased ? No ; this does not often 

 happen ; but the effect of astigmatism has doubled, and 

 this has happened in the following manner : — An eye 

 which is nonnal as regards the vision of vertical lines, 

 but short-sighted for horizontal lines, sees the objects 

 elongated in a vertical direction. When the time of life 

 arrives that the normal eye becomes far-sighted, but not 

 yet the short-sighted eye, this astigmatic eye will at short 

 distance see the vertical lines indistinctly, but horizontal 

 lines still distinctly, and therefore near objects elongated 

 in a horizontal direction. The portrait painter, in whom 

 a slight degree of astigmatism manifested itself at first 

 only by the indistinctness of the horizontal Unes, has now 

 become far-sighted for vertical lines, therefore he sees a 

 distant person elongated in a vertical direction ; the por- 

 trait he paints, on the contrary, being at a short distance, 

 is seen enlarged in a horizontal direction, and thus painted 

 still more elongated than the subject is seen ; so the fault 

 is doubled. 



The lecturer proved these remarks by showing a picture 

 which he made to appear in its natural shape or distorted 

 by elongation, in either a vertical or a horizontal direc- 

 tion, by means of a lens which he held at various dis- 

 tances from the optical apparatus. 



■ in. — MULREADY 

 The lens, continued the lecturer, always gets rather 

 yellow at an advanced age, and with many people the in- 

 tensity of the discoloration is considerable. This, how- 

 ever does not essentially diminish the power of vision. In 



order to get a distinct idea of the effect of this discolora- 

 tion, it is best to make experiments v.-ith yellow glasses of 

 the corresponding shade. Only the experiment must be 

 continued for some time, because at first everything looks 

 yellow to us. But the eye soon gels accustomed to the 

 colour, or rather it becomes dulled with regard to it, and then 

 things appear again in their true light and colour. This 

 is at least the case with all objects of a somewhat bright 

 and deep colour. A more careful examination, however, 

 shows that a pale blue, or rather a certain small quantity 

 of blue, cannot be perceived even after a very prolonged 

 experiment, and after the eye has long got accustomed to 

 the yellow colour, because the yellow glass really excludes 

 it. This must, of course, exercise a considerable influ- 

 ence when looking at pictures, on account of the great 

 difference which necessarily exists between real objects 

 and their representation in pictures. 



These differences are many and great, as has been so 

 thoroughly explained by Helmholtz. Let us for a moment 

 waive the consideration of the difference produced by 

 transmitting an object seen as a body upon a simple flat 

 surface, and let us only consider the intensity of light and 

 colour. The intensity of light proceeding from the sun 

 and reflected by objects is so infinitely greater than the 

 strongest light reflected from a picture, that the propor- 

 tion expressed in numbers is far beyond our comprehen- 

 sion. There is also a great difference between the colour 

 of light or of an illuminated object, and the pigments 

 employed in painting, and it must appear wonderful that 

 the art of painting can produce by the use of them such 

 perfect optical delusions. It can, of course, only produce 

 optical delusions, never a real optical identity ; that is to 

 say, the image which is traced in our eye by real objects 

 is not identical with the image produced in our eye by 

 the picture. 



Returning to our experiment with the yellow glass, we 

 shall find that it affects our eye very much in the same 

 way as a yellow tint of light. The small quantity of blue 

 light which is excluded by the yellow glass produces no 

 sensible difference, as the difference is equalised by a 

 diminution of sensibility with regard to yellow. In the 

 picture, on the contrary, there is found in many places 

 only as much blue as is perfectly absorbed by the yellow 

 glass, and this therefore can never be perceived, however 

 long we continue the experiment. Even for those parts 

 of the picture which have been painted with the most 

 intense blue the painter could produce, the quantity of 

 blue excluded by the yellow glass will make itself felt, 

 because its power is not so small with regard to pigments 

 as with regard to the blue in nature. 



With aged people we often find the crystaUinc lens to 

 be of a yellowish tint. In pictures painted after the 

 artists were over sixty, therefore, the effect of the yellow 

 lens can often be studied. As a striking example, the 

 lecturer mentioned Mulready. It is generally stated 

 that in his advanced age he painted too purple. A more 

 careful examination shows, however, that the peculiarity 

 of the colours of his later pictures is produced by an addi- 

 tion of blue. Thus, for instance, the shadows on the 

 flesh are painted in pure ultramarine. Blue drapery he 

 painted most unnaturally blue. Red of course became 

 purple. If we look at these pictures through a yellow 

 glass all these faults disappear ; — what formerly 

 appeared unnatural and displeasing is at once corrected ; 

 the violet colour of the face shows a natural red ; the 

 blue shades become grey ; the unnatural glaring blue of 

 the drapery is softened. It happens that Mulready has 

 painted the same subject twice, first in the year 1836, 

 when he was fifty years of age and his lens was in a 

 normal state, and again in 1857, when he was seventy- 

 one and the yellow discolouring had already considerably 

 advanced. The first picture was called when exhibited 

 " Brother and Sister ; or. Pinching the Ear ;" the second 

 was called " The Young Brother." If we look at the 



