TRANSACTIONS OF THE SECTIONS. 



portion of red light as distinctly as an almond placed beside a cherry. This is an 

 experimentum crucis, if one were wanting in favour of the doctrine of a triple 

 spectrum, — of the existence of three colours, red, yellow, and blue at the same point 

 of the spectrum. 



On the Binocular Vision of Surfaces of Different Colours. 

 By Sir David Brewster, K.H., F.R.S. L. <Sf E. 



Prof. Dove had published an account of some beautiful expnriments in connexion 

 with this subject some years ago. M. Dove showed in his paper, that when dif- 

 ferent colours at the same real distance are regarded by the eye, they appear to be at 

 different distances ; this is also the case when a white surface is compared with a 

 black. Now M. Dove argues if a white surface and a black one be stereoscopically 

 combined, one of them must be seen through the other. Taking a figure for the left 

 eye with a white ground, and a second figure of the same object on a black ground 

 for the right eye, when these two figures are combined, a beautiful effect is observed; 

 the figure starts into relief, and its sides appear to possess a shining metallic lustre. 

 This is the case when the surface of each single object is quite dull and without lustre. 

 On this experiment M. Dove founds a theory of lustre, supposing it to be produced 

 by the action of light received from surfaces at different distances from the eye. 

 An example of this is the effect observed on looking at varnished pictures : one por- 

 tion of the light comes from the anterior surface of the varnish and the other from 

 its posterior surface, the action of both of these conspiring to produce the observed 

 lustre. The metallic lustre of mica is also referred to by M. Dove as an example 

 of the same kind. In his present communication, Sir David Brewster controveits 

 the theory here laid down, and bases his objections on the following remarkable ex- 

 periment : — where a white surface without definite boundary and a black surface of 

 the same kind are regarded through the stereoscope, no lustre is observed. Sir David 

 therefore infers that the lustre is due, not to the rays from one surface passing 

 through the other to the eye, but to the effort of the eyes to combine the two stereo- 

 scopic pictures. 



On the Existence ofAcari in Mica. By Sir David Brewster, K.H., F.R.S. 



While examining with a microscope a thick plate of mica from Siberia, about 5 inches 

 long and 3 inches wide. Sir David was surprised to observe the remains of minute 

 animals, some the 70th of an inch, and others only the 150th of an inch in size. 

 Some of these were enclosed in cavities, round which the films of mica were in optical 

 contact. These acari were, of course, not fossil, but must have insinuated themselves 

 through openings between the plates of mica, which afterwards closed over them. 



On the Absorption of Matter by the Surfaces of Bodies. 

 By Sir David Brewster, K.H., F.R.S. L. ^ E. 



If we smear, very slightly, with soap the surface of a piece of glass, whether arti- 

 ficially polished or fused, and then clean it perfectly with apiece of chamois leather, 

 the surface, when breathed upon, will exhibit, in the most brilliant manner, all the 

 colours of thin plates. If we breathe through a tube, the colours will be arranged 

 in rings, the outermost of which is black, corresponding to the centre of the system 

 of rings formed between a convex and a plane surface of glass. In repeating this expe- 

 riment on the surfaces of other bodies. Sir David found that there were several on whose 

 surfaces no colours were produced. Quartz exhibited the colours like glass, but cal- 

 careous spar and several other minerals did not. In explaining this phsenomenon, the 

 author stated that the particles of the soap, which are dissolved by the breath, must 

 either enter the pores of the bodies or form a strongly adhering film on their surface. 

 This property of appropriating temporarily the particles of soap, becomes a new di- 

 stinctive character of mineral and other bodies. 



On the Remains of Plants in Calcareous Spar from Kings County, Ireland. 

 By Sir David BreWster, K.H.,' F.R.S. L. Sf E. 



