Fkbbvary 1, 1901. 



KNOWLEDGE 



•ir. 



quakes. Seismological committees or departments have 

 beeu established in several countries. Thanks to the 

 work of Milne and Omori in Jap;ui, of SadciTa and 

 Coronas in the Philippines, of do Rossi, Agamcnnone, 

 Bai-attA and Mercalli in Italy, of Eginitis and Papar 

 vasiliou in Greece, and many othei^s, some of the finest 

 seismic regions in the world are now secure from 

 neglect. To the laboiu-s of von Rebeur-Paschwitz, Milne, 

 and Gerland. in founding a seismic .survey of the world, 

 we may look forward with ronfidcnce to obtaining a 

 rich harvest of results. 



(7) ConcuiTcntly with the growth in our knowledge, 

 the origin of earthquakes has become more clearly 

 understood. There are many shocks, marked as a rule 

 by small disturbed areas and abnormal intensity near 

 the centre, which we can hai'dly err in attributing to 

 volcanic action in some cases, and in others to local 

 disturbances partly natural and partly artificial. But 

 all severe earthquakes, and the majority of slight ones, 

 we seem to be equally justified in connecting with the 

 formation of faults. In regai-ding earthquakes as the 

 pa,ssing cffcctf; of the gradual but intermittent growth 

 of faults, wc are relying on a source of energy competent 

 to produce the strongest as well as the weakest shock. 

 At the same time, we are investing earthquakes witli 

 a significance which they certainly did not possess for us 

 at the beginning of the nineteenth century, as indices 

 of the site and epoch of the changes that are now taking 

 place in the eiarth's crust. 



/ ' 



Conducted by M. I.Crpss 



Photo-Micrography.— One of the great difficulties experi- 

 enced by novices in photo - micrography is in the attempt 

 to get an evenly illnminated disc with a clear sharp edge. 

 Various devices have been tried to accomplish this, including 

 masks placed immediately in front of the plate during exi)osure, 

 and cuitiag down the disc by means of a mask when printing 

 from the negative, but these are really, so far as the disc is 

 concerned, attempts to obliterate an original error of manipula- 

 tion. The necessity for masking off a portion of the field 

 during printing so as to get only the portion that is in focus in 

 the centre is not now under consideration. 



It is proposed to give directions which, if attended to, will 

 ensure a clear, evenly illuminated disc with a sharp edge. 



In the first place suitable apparatus is requisite, and in addi- 

 tion to the camera, microscope and objectives, the following 

 should be included : illuminant — preferably zirconium or lime 

 light — tjuU's-eye condenser vith irh (liaiihrucfiii immediately 

 in front of it ; substage condenser ; projection eyepiece ; and a 

 piece of glazed cardboard about G inches square mounted on a 

 wooden support so that it may be central with the optic axis of 

 the microscope ; a darkened room. 



The microscopic object should be placed on the stage and the 

 objective and substage condenser adjusted exactly as for a 

 visual examination. The microscope is then set in position on 

 the camera base. The cardboard should now be set up at a 

 distance of about G inches from the eyepiece. 



Light up, centre the jet or .source of illumination as nearly as 

 can be roughly done to the buirs.e3'e, then centre the buU's-eye 

 to the microscope. This is done in the following manner : — 



It will be observed that a faintlj* illuminated di.sc will appear 

 upon the cardboard when light is passed through the condensers. 



The iris diaphragm attached to the buU's-oye must now Ije 

 treated as the source of light, and the ])osition of the bull's-eye 

 altered until the centre of this iris diaphragm corresponds with 

 the centre of the micro.scope. In order to do this exactly, the 

 iris of the bull's-eye should bo almost closed, and if the focus of 

 the substage condenser be altered slightly the outline of this 

 iris diaphragm will appear sliarply upon the card. It sliould, 

 after accurate centering, be opened to just such an extent as the 

 circumstances allow. Next alter the adjusting collar carrying 

 the eye-lens of the jirojection eyepiece so that a sharply defined 

 edge is given to the disc. 



It will now be necessary to centre the light to the bull's-eye, 

 which together with its backward oi- forward position can be 

 checked by allowing its image to fall upon a screen of card set 

 at the back of the substage condenser, in the manner recom- 

 mended in the various text-books. 



If all these adjustments are made with care, and the 

 iris of the bull's-eye opened to tin: proper extent, an evenly 

 illuminated disc should result, and nothing remains but to view 

 and finally focus the image upon the focussing screen and take 

 the photograph, using of course such coloured glasses, etc., as 

 the objects may necessitate. 



A New He.\tini; Staue. — Mr. Leonard I'. Wilson, r.r.s., 

 sends us the following description of a new heating stage of his 

 own design. During the course of some recent crystallognipliic 

 investigations, it was considered necessary to examine, micro- 

 scopically, a number of substances at known constant tem- 

 peiutiiros. In order to do this, a heating stage was essential, 

 but none of tliose in geneial use appeared to fulfil the require- 

 ments, the fault lying chiefly in the tact that the slide, being 

 heated only from below, was cooled irregularly by currents of 

 air. A stiige was therefore designed in which the temperature of 

 the slide could be accurately known, and could be easily con- 

 trolled. The construction of this stage was such that the 

 heating medium, generally water, flowed both above and below 

 the slide, thus heating it uniformly, while air currents could 

 only enter by the narrow aperture through which the slide and 

 its carrier were introduced into the stage, the circular apertures 



Fig. 2. 



in the optic axis of the microscope being protected as hereafter 

 described. As shown in Fig. 1, the stage is in the form of 

 a double box, forming a water jacket to the .slide audits carrier. 

 Water of the required temperature is passed in at A, and flows 

 out at B, the temperature being taken by means of a 

 thermometer inserted into the tube C, round which the water 

 passes in the middle of its course. The heating stage is 

 fastened to the mechanical stage by means of the screw D. 

 When using a high power, the objective passes into the upper 

 a])erture of the stage, and is below the level of its upper surface, 

 and in order that the stage may move freely in all directions 

 in conjunction with the movements of the mechanical stage, it 

 is necessary tliat this tubular aperture be greater in diameter 

 than the objective. To prevent the intrusion of cold air which 

 might take place owing to this difference in diameter, a vulcanite 

 plate, having an aperture which the objective just fits, slides on 

 the suifaoe of the stage, under the clips L,E. In a manner 

 similar to the objective, the condenser pas.ses up into the lower 

 portion of the stage ; space for the vulcanite plate in this case 

 licing made by the introduction of two bars X, X, across the 

 lower surface of the statue, which is thereby raised above 

 the mechanical stage. The slide carrier, as shown in Fig. 2, 

 consi.sts of a brass plate, having a circular aperture through it, 



