PROCEEDINGS OF THE SOCIETY. 109 



curvature seem only to be possible by the creation of new optical materials. 

 The use of nltra-violet rays admits of increasing the definition to a 

 considerable degree ; but the insufficient transparency of media fre- 

 quently imposes a limit. 



" A New Microscope Illuminator," by Mr. Alexander Silverman. 



The illuminator described is largely used in America. It is claimed 

 to show greater detail than older forms when examining opaque objects, 

 sucJi as metals, and it is of special value for papers, textiles, etc., which 

 are invisible under vertical light. 



" Some Problems in High Power Photomicrography," by Dr. R. E. 

 Slade, F.I.C, and Mr. G. I. Higson, M.Sc. 



The paper describes devices to ensure the greatest possible resolving 

 power in examining photographic emulsions. The source of light is a 

 " Pointolite " lamp. No optical system — merely a colour screen for 

 reasons explained — is interposed between the lamp and the condenser. 

 A vibrationless shutter operates in front of the eyepiece of the micro- 

 scope ; no camera is employed. 



Mr. R. J. E. Hanson, F.R.C.S., contributed a paper on " Fatigue 

 Factors Incidental in the Use of Certain Optical Instruments," which 

 was taken as read. 



Defects in or moisture of the muscular mechanism of eye-movement 

 are considered, as they concern the microscopic observer. A head-piece 

 is described which obviates fatigue, which can be attached to any standard 

 microscope. 



A group of papers dealt with the subject of " Glass for Optical 

 Purposes." 



Dr. Morris W. Travers, F.R.S., referred to the work done in 

 America in 1917, when a sudden demand arose for an enormous supply 

 of optical glass. No information existing in this country was obtainable 

 on that occasion, but by June 1918 a group of twenty scientific men 

 ' from the Geophysical Laboratory and the Bureau of Standards co- 

 operating with the industry succeeded, after two months' concentrated 

 effort, in producing the quantities required and in qualities that seldom 

 called for rejections. He deprecated the Government policy in this 

 country of attempting to monopolise science. 



Dr. W. E. S. Turner dwelt on the difficulties involved in making 

 optical glass, the demand for which was so small, and he suggested 

 remedies for meeting them. He considered England could supply all 

 her own needs, and he thought America behind this country in output 

 and variety ; indeed, there was a market for glass made here. 



He forwarded to the meeting a beautiful specimen of a crystal of 

 calcium fluoride from Johannesburg. 



Mr. Robert Mond read a note relating to the occurrence of fluorite 

 in Canada, and he submitted a specimen from the one deposit that 

 showed any promise. A lens would be cut from it to test its optical 

 properties. 



