39 



denser. The lecturer read au extract from a paper 

 read by him before the Royal Society at this time, detailing 

 his own investigations and experiments, and which proved 

 that to him was due the credit of discovering the mode of 

 producing dark ground illumination by oblique transmitted 

 light at a certain definite angle, which he illustrated by 

 diagTauis aud the microscope. He then adverted to themethod 

 pursued by himself, in conjunction with Dr. Carpenter, in 

 obtaining dark ground illumination, aud explained with 

 much minuteness the way in which his experiments were 

 conducted, and the success which attended them. The 

 labours of Mr. Ross and Mr. Shadbolt were then referred 

 to— particularly the spot lens of the former, which was an 

 achromatic condenser, with a small spot to stop out the 

 central rays of light, aud admit only those beyond the angle 

 of vision— and the paraboloid of the latter— which was 

 shown and explained by the lecturer. The condensers of 

 Smith, and Beck, and Gillet were next passed under review, 

 and shown both in use and on the black-board. The lecturer 

 then referred to, and explained the construction of a 

 magnificent microscope which was presented to him by his 

 late parishioners at Stone. After he had it in his posses- 

 sion some time, he thought on a mode by which the con- 

 denser might be made more useful, and this led to a great 

 improvement on the solar microscope. The lecturer then 

 described at great length, and with much minuteness, the 

 mode by which the kettledium condenser (an invention of 

 his own) was used for conveying the rays of light in in- 

 struments of high po wer, by admitting only two pencils of 

 rays at angles of 30, 60, or 90 degrees apart, thus throwing 

 into prominent relief all projections or lines crossing each 

 other at all or either of those angles. He then 

 spoke of the application of polarised light to mi- 

 croscopical illumination, and explained the curious and 

 interesting phenomena of this wonderful discoveiy. 

 The undulatory theory of Dr. Young accounted 

 in a very satisfactory manner for all these phe- 

 nomena ; and he therefore thought that this theory of ligh^ 



