748 PKOCEEDINGS OF THE SOCIETY. 



Dr. Hebb also exhibited a filter devised by Mr. Taverner for obtaining 

 clear mounting and other media, and read a description of the apparatus. 

 The thanks of the Meeting were voted to Dr. Hebb for bringing 

 these matters before the Society, and to Messrs. Doulton and Taverner 

 for their exhibits. 



Mr. Conrad Beck exhibited an optical bench, which having been 

 shown on a former occasion he did not think it necessary to describe. 

 It was on that occasion fitted with a Nernst lamp with single filament, 

 and a diffraction grating, giving a brilliant spectrum, any particular 

 portion of which could be admitted to a Microscope by altering the posi- 

 tion of a slit cut through an opaque screen placed in the path of the 

 beam. In this instance it was arranged to show experimentally how 

 Amphiphura pellucida could be resolved by the green light, whilst the 

 yellow was unable to do this. 



The thanks of the Meeting: were voted to Mr. Beck for this exhibit. 



Mr. Poser exhibited a special pattern Microscope, constructed by 

 Messrs. Zeiss, chiefly for photomicrography in metallurgical work,* and 

 the thanks of the Meeting were given to Messrs. Zeiss for sending this 

 for exhibition. 



Mr. Gordon read his paper, " On the Use of a Top Stop for Developing 

 Latent Powers of the Microscope," and exhibited and briefly described his 

 apparatus — which had been previously shown to the Society in a rather 

 less complete form, and is described in this JournaLf He then pointed 

 out that a top stop enables the microscopist to vary the proportion be- 

 tween the refracted and the unrefracted light which passes the instru- 

 ment, and thus to render conspicuous a particular feature of the object, 

 and to accomplish this with great nicety. In illustration of the results 

 thus reached he exhibited a number of photographs taken with an 

 achromatic oil immersion objective of N.A.= 1. A series of pictures of 

 Pleurosigma formosum showed how the features of an image change 

 with change of aperture, and a photograph of P. angulatum showing 

 the "white dot growing up within the black dot," was exhibited to 

 demonstrate how, by means of a top stop, the objective in question 

 could be made to equal the performance of an objective of much wider 

 aperture. Finally a photograph of Staphylococcus was exhibited in 

 which the stained spherules, characteristic of the object, were seen held 

 together by a little mass of unstained enveloping jelly, very conspicuous 

 and unmistakable in the image formed by the stopped beam, but barely 

 discernible in the image formed by an unstopped beam, even with a 

 lens of the highest power and largest numerical aperture. In the course 

 of reading the paper he announced that Mr. Crouch had just shown him 



* This Microscope was described and illustrated in the Journal for October, 

 pp. 598-600. f See ante, p. 157; and for diagram, p. 365. 



