434 TRANSACTIONS OV SECTION C. 



perhaps, as zinc is commonly accompanied by cadmium. For these reasons I feel 

 that the lead-ratios of thorium minerals, notably those of the thorites and 

 thorianites of Ceylon, and the thorites of the Devonian pegmatites of southern 

 Norway, should be altogether rejected for our purpose. 



At the present time, the radioactive problems wliich, for the geologist, are 

 in most urgent need of solution are concerned with the possible dependence of 

 the rate of decay of uranium on time, pressure, and temperature. In turn, the 

 geological problem now most likely to throw light on the correct interpretation 

 of lead-ratios is that of deciding whether or not the earth has ever been molten 

 at or near the surface. 



(c) Coiiiribution to Discussion on Radio-active Problems. 

 By Professor F. Soddy, F.R.S. 



Professor Soddy hoped that geologists would not be in any immediate hurry 

 to decide between the geological and radio-active estimates of the age of the 

 earth. Owing to the element of luieertainty about the initial stages of the 

 disintegration and the long periods involved there was a gTeat terra incognita, 

 and the new theory of isotopes made it necessary to take into account many 

 possibilities, some of them referred to by Professor Joly, not thought of a 

 couple of years ago. In addition there was always the possibility that thorium 

 might be a branch of the uranium family, in which case some of the argument.'! 

 that had been used entirely fell to the ground. While he saw no successful 

 method at pre.sent of altering the (/eneral order of the radio-active estimate, 

 he did not regard it as more than tentative, and there might well be unknown 

 factors still to be discovered sufficiently important to bring the two methods 

 into closer agreement. 



9. Twinning in Metallic Crystals. By Professor 0. A. Edwards. 



10. The liolatioi of th.c Direclions-Lmage of a Mineral in a Rock-Slice. 

 By J. W. Evans, D.Sc. LL.B. 



The author discussed the different methods by which the interference figures 

 of a small mineral in a rock-slice may be kept distinct from those of adjoining 

 minerals. He recommended two. In one, which he believes to be new, a 

 diaphragm with a small aperture is placed below the condenser, which is lowered 

 till the image of the aperture appears in focus on the rock-slice. In some micro- 

 scopes the iris diaphragm provided for the Becke method of determining the 

 refractive index may be employed. In others it is too near the condenser. The 

 aperture should be sufficiently large to illuminate the maximum area of the 

 mineral under investigation, but no poi-tion of the others. The directions-image 

 may then be observed in any of the usual ways. Unless the condenser and 

 diaphragm revolve with the stage the aperture must be very carefully centred 

 with the axis of rotation. 



The other method was proposed by Becke in 1895, but is very little known. 

 The diaphragm is placed in the focus of the eye-piece so as to shut out all 

 except the mineral selected. The Becke lens, or system of lenses resembling an 

 eye-piece, is placed above the eye-piece, when the directions-image of the mineral 

 will be seen without any admixture of light from its neighbours. This method 

 has the advantage that the diaphragm is less highly magnified at the time of 

 adjustment. When a rotating stage is employed, a very accurate centring of 

 the nose-piece of the microscope is required, so that the coincidence of the object 

 with the aperture may be maintained. 



The common practice of placing a diaphragm for this purpose immediately 

 below the Bertrand lens rests on no scientific basis, and is not effective in 

 sliutting out the light of minerals other than that which is being studied. 



