358 ritOCEEDIXGS of the SOl'IETY. 



principle was enforced by reference to the so-called temporary blinding 

 of the eye by sudden changes of luminosity, as, for example, from — 



1. The sun . . . .to Bright daylight, 



2. Bright daylight . . . ,, Interior daylight, 



d. Artificial illuminants themselves „ Interior artificial lighting, 

 4. Interior artificial lighting . ,. The photographer's darkroom, 



although prolonged observations without strain or discomfort could be 

 conducted in most of these kinds of lighting when the eye was not called 

 upon to make violent adjustments by changing over from one kind to 

 another. 



Mr. Maurice Blood commented upon the importance of this con- 

 tribution with refei'ence to the accui-acy of range-finders. The im- 

 portance of keeping the luminosity of the microscopical field and that 

 of the worker's surroundings of equal intensity had been impressed upon 

 him by long hours of work, and his own practice was to darken the 

 room somewhat, to cut down the illumination of the microscopical 

 field, and, finally, the size of the field itself by means of an iris-diaphragm 

 ill the eye-piece. 



Mr. Ainslie, R.N., in estimating the importance of the factors 

 affecting accuracy in the use of range-finders at sea, considered that 

 more importance must be attributed to the motion of the ship than to 

 varying luminosities, and, further, regarded contrasts between the 

 colour in the field of view and that of the general light as of much 

 more importance than variation in the intensity of the luminosity. 



Sir E. Ray Lankester described a device by which the luminosity 

 of an electric arc was reduced to that of ordinary diffuse day-light, and 

 stated that the observation of an artist friend proved that such light 

 was exceptionally useful for prolonging the hours of work when using 

 oil colours. He inquired whether it was within the knowledge of any 

 of the Fellows that such illumination had been used for the purpose 

 of microscopy. 



Mr. Barnard remarked that he had had the advantage of reading 

 the paper in advance, and had therefore made an experiment to test the 

 method suggested under working conditions. There was perhaps no 

 more difficult image to focus than the fluorescent one which resulted 

 from the use of ultra-violet light in photomicrography. If the room in 

 which this work was being carried on was illuminated by !means of 

 monochromatic green light, as emitted by a mercury vapour lamp, it 

 appeared to be easier to focus the fluorescent image than under any other 

 conditions. His experimental observations in this connexion, therefore, 

 supported the contentions of Mr. Purkiss. 



A paper by the Rev. Hilderic Friend on "AHen Oligochjets in 

 England " was taken as read, and will appear in full in the Society's 

 Journal. 



