PROCEEDINGS OF THE SOCIETY. 535 



Olausius thus incidentally proved many years ago that any good 

 condenser gives the brightest possible flame-image, and that any optical 

 addition whatever to such a condenser must reduce the brightness of the 

 flame-image, owing to absorption and other losses in the addition. 



As in the arrangement proposed by Mr. Nelson more light un- 

 doubtedly enters the condenser than would enter without the added 

 glass ring, the question remains to be answered : What becomes of this 

 light ; why is the flame-image not rendered brighter by it ? It is rather 

 surprising that Mr. Nelson did not discover the answer himself, for, on 

 elementary considerations of the same type as those which led to the 

 fixing of the Gauss principal and cardinal points, and to the definition of 

 equivalent focal length, it is immediately obvious that the equivalent 

 focal length of the condenser is increased by the addition of the glass 

 ring in the exact proportion which the diameter of the pencil entering 

 the glass ring bears to that which enters the condenser itself. Supposing 

 this proportion to be as 2 to 1, then, as Mr. Nelson pointed out, twice as 

 much light enters the glass ring as that which would get into the con- 

 denser direct. But simultaneously the focal length of the condenser, and, 

 therefore, the linear dimensions of the flame-image, are doubled, and the 

 area of the flame-image is therefore increased fourfold. We therefore, 

 with the ring interposed, get twice the quantity of light spread over four 

 times the area ; in other words, the intrinsic brilliance of the flame- 

 image, instead of being doubled, is actually halved by the interposition 

 of the ring. The reason why there is not only no gain, but an actual 

 loss of brilliance is that the condenser fitted with the ring obviously does 

 not fulfil the sine-condition. 



Mr. E. B. Stringer's note, " On the Use of the Mercury Vapour 

 Lamp in observing the Rings and Brushes in Crystals," was read by 

 Dr. Hebb. 



Mr. C. L. dirties said that in further illustration of this paper he 

 had brought to the Meeting two photomicrographs to show the difference 

 in the appearance of a crystal of calcite under the two means of illumina- 

 tion, by which it would be seen that with the mercury vapour lamp the 

 rings extended quite out to the edge of the field. He also exhibited the 

 apparatus in operation, arranged so that a crystal of calcite would be 

 illuminated alternately by the Nernst lamp and by the mercury vapour 

 lamp. 



A communication from Mr. E. B. Miller- Williams, "On a New 

 Fine-adjustment for Body and Substage of Microscopes," was also read 

 by Dr. Hebb, and was illustrated by coloured drawings of the proposed 

 device, which were passed round for inspection. 



Dr. Hebb said the drawing apparently indicated an old device known 

 as . the screw-in-the-post system, but without being able to inspect a 

 working model in action it would be unwise to pass an opinion as to the 

 merit of the invention ; still, it was very gratifying to have the benefit 

 of seeing Mr. Miller- Williams's ingenious design even only as a schematic 

 representation, and it was to be hoped that the inventor would see his 

 way to exhibit later an effective working model. 



