41 



less elliptical — the effect was as if it were pressed laterally and so made ellip 

 tical. A necessary consequence of this was that dots which were very near to 

 each other would be seen in one direction as a line, from their being converted 

 into ovals and overlapping each other. Another frequent error of interpre- 

 tation was as to whether a thing was within or outside the focus, and no 

 two persons seemed to agree as to what was exactly the focus. A blond disc 

 for instance, on one side of the focus appeared to be concave, whilst it did not 

 appear so on the other side, and it was a question with many which was the 

 reality. Then again there were the markings on Pleurosigma formosum. Some 

 persons saw them as convex hemispheres and others thought them concave, and 

 they were frequently seen to be more or less coloured for reasons which were 

 entirely personal. Another question arose with regard to Micrometry— a long 

 and a short sighted person would use the Micrometer, and their observations 

 would not agree on account of their differences of focus ; whilst some persons 

 see shadows and cannot tell accurately which is the margin of the object. As 

 regarded the question of fluorescence, it was a very curious fact that when a 

 person had been saturated with quiniue (given not in small quantities as a tonic, 

 but in large quantities in cases of ague, as an antiperiodic medicine) the effect 

 was generally headache and a confusion of vision, and they were able to see 

 much farther towards the violet end of the Spectrum than under normal con- 

 ditions. But in such cases it was found that vision was impaired in exactly the 

 same proportion towards the red end, so that their range was not really 

 increased. It should be borne in mind in connection with the subject of colour 

 that all their microscopes were corrected for white light, but they were for the 

 most part used in very yellow light, and they could not suppose that the effects 

 under these conditions would be the same as in day light, and it was not possible 

 to get by ordinary means within the reach of every one monochromatic light, 

 strong enough for microscopical purposes. Irradiation, also, must not be over- 

 looked, as being a frequent source of personal error; for even in landscape 

 photography there was found to be an irradiation effect produced, which be 

 came less marked when the back of the plate was smeared over with yellow. Its 

 common effect was that 999 people out of 1000 could see light lines upon a dark 

 ground better than they could see dark lines upon a light ground, and this 

 would account for the better definition commonly produced by dark ground 

 illumination— either by the paraboloid, or by oblique illumination from the 

 mirror. The former of these methods was, however, much preferred, owing to 

 the diffraction which was frequently caused when the other was adopted. If a pei- 

 son were temporarily ill he could not see things in the same way as when in health 

 and it was quite certain that the presence of bile where it should not be must 

 tend to a disturbance of vision. He hal at that moment a person under his 

 care who was dying from cancer of the liver, all of whose tissues and fluids 

 were so saturated (and coloured) with bile that even the saliva was much tinged, 

 and very bitter. In this case there was much confusion of vision, accompanied 

 by spectra— probably owing to alteration in the density and colour of the 

 humors of the eyes. It had even been asserted by some observers that objects 

 seemed yellow to such sufferers, and the same had been observed by those who 

 had taken Santonica medicinally. 



Mr. Ingpen said, in reply to Mr. Waller's remarks, that probably Professor 

 Liebreich knew nothing of the special circumstances with regard to Mulready, 

 but simply drew his conclusions from observation of the paintings themselves 

 The fact of yellow cornea was very pertinent to the subject, and it was also 

 quite possible that some artists might not have been affected in that way. With 



