112 .Contributions to the Physiology of Vision. 



VIII. Dr. Purkinje has made numerous experiments on 

 what are generally called ocular spectra, viz., the images 

 which remain after objects have been regarded for some time; 

 he gives a more detailed account of them than we believe is 

 elsewhere to be found, and the following extract will, we trust, 

 be read with interest. 



1. Looking stedfastly for a very short time at the flame 

 of a candle, then quickly covering the eye, the bright image 

 of the flame remains, but instantly disappears from the cir- 

 cumference towards the centre, leaving a red shining flame, 

 which becomes invisible in the same manner, and is replaced 

 by a white image ; this also, though rather slowly, fades away, 

 and after having completely disappeared, leaves a dark coloured 

 contour of the flame with a greyish nimbus, which ultimately 

 enlarges towards the centre, and thus terminates the whole 

 appearance. If, at the beginning of the experiment, the eye, 

 instead of being covered, is directed towards a white surface, 



manner : place as near to the eye as possible a plate of ground glass, and upon its 

 external surface lay a card, in which a large pinhole has been made ; adjust this 

 aperture so that it shall be in a right line drawn from the eye to the flame of a 

 caudle. When the card is kept in motion so as to displace continually the image 

 of the light in a small degree (vide Scheiner's experiments), the veins will be seen 

 distributed as above, but they will now appear brighter than before, and 

 the spaces between the ramifications will be seen filled with innumerable minute 

 tortuous vessels, which were in the former experiment invisible : in the very centre 

 of the field of vision there is a small circle, in which no traces of them appear, 

 and in the centre of this circle is seen a darker point. The same appearance will 

 be seen by moving a pinhole close to the eye when looking at a ground-glass 

 window-pane, an illuminated white wall, or a sheet of paper. The absence of 

 vessels at the centre of the retina will probably account for the greater distinctness 

 with which small objects are there seen, and also for the difference of colour ob- 

 served by anatomists in that part of the nervous expansion. 



The following experiment, as well as the preceding, is original, both having 

 been observed by myself in attempting to verify the discoveries of Purkinje. 

 In the ordinary circumstances of vision, particles floating in the humours of the 

 eye, or specks in the cornea or crystalline lens, are invisible, because their shadows 

 are projected by different rays of light on every part of the retina, thus permitting 

 no distinct image of them to be formed ; but they may be rendered visible by 

 allowing only a single ray of light to fall on the eye through a hole made in a card 



by a very fine needle, and placing the light and aperture so that the object within 

 the eye may be in a right line with them and the centre of the retina : they may 

 be projected on any part of the retina, but they will be most distinctly seen at this 

 point of most perfect vision. I have thus observed, in my own eye, collections of 

 transparent globules which, from their free motions, evidently exist in the hu- 

 mours ; and one remarkable spot (in my left eye), which, from its permanence, 

 must be either in the cornea or the lens : after winking, the secretion Irom the 

 lacrymal ducts is also very obvious. 



These experiments may probably afford to the oculist a means of ascertaining, 

 from the direct observation of his patient, various morbid changes in the retina and 

 lenticular apparatus of the eye. C. W. 



