154 



NATURE 



[June 14, 1900 



■disruptive discharge would take place in air, and could only 

 be obtained in vacuo. In a footnote, however, Becquerel 

 tells us that he has since observed the electric deviation in 

 air with a field of about lo^^ C.G.S. units, and has found 

 for certain rays which pass through black paper the 

 values ;«/^= lo^ and v= i"6 x iqI*'. 



The chemical effects of Becquerel rays have been 

 examined by M. and Mme. Curie and Becquerel ; they 

 may be briefly summarised here. The rays from active 

 •salts of barium transform oxygen into ozone, a process 

 involving a continuous expenditure of energy. Potassium 

 iodide is coloured blue. Glass in contact with the salts 

 is coloured violet, ultimately becoming nearly black, and 

 the colour penetrates the glass ; this phenomenon is 

 analogous to the coloration of fluorspar by kathodic 

 rays. Platinocyanide of barium screens gradually turn 

 yellow, then brown, and finally lose their fluorescence, 

 which, however, is restored by exposure to sunlight. 

 Fluorine continues. to phosphoresce for twenty-four hours 

 after being excited, and calcined fluorspar which has lost 

 its phosphorescence regains its luminosity in the presence 

 •of radium. Chemical activity is confined to those radio- 

 active preparations which are luminous, but is not always 

 proportional to the luminosity. 



According to the Curies' experiments, powerfully radio- 

 active compounds of radium and polonium^ when they 

 act on inactive substances, are able to communicate radio- 

 activity to them. This induced radio-activity increases 

 with the time of exposure up to a certain limit. If the 

 inducing substance is 5000 to 50,000 times the activity of 

 -uranium, the induced activity may amount to fifty times 

 that of uranium. It is reduced to one-tenth of its amount 

 in an hour after removal, but it may persist for many 

 •days, finally disappearing. The emanation of radio-active 

 particles from thorium compounds, investigated by Ruther- 

 ford, is remarkable. This emanation ionises the gas in 

 its neighbourhood, and it will pass through thin layers of 

 metal, through thicknesses of paper, or through a plug 

 of cotton wool. It is also unaffected by bubbhng through 

 hot or cold water, weak or strong sulphuric acid. The 

 emanation retains its radio-active power for some minutes, 

 gradually losing it. The positive ion produced in the gas 

 by the emanation was found to possess the power of 

 inducing radio-activity in all substances on which it fell, 

 this power of giving radiation lasting several days. 

 Whether the emanation be a vapour of thorium is doubtful. 



The question as to the amount of energy emitted by the 

 Becquerel rays has already been referred to in Nature, 

 and need not therefore occupy our space further now. 

 The problem of discovering the seat of this energy 

 would seem of late to have taken another form. At first 

 it was supposed that a difficulty would exist in reconciling 

 the continuous emission of these rays with the principle 

 of conservation of energy ; now, however, that the 

 amount of the emitted energy has been estimated, the 

 difficulty is seen to lie in the experimental observation of 

 changes of such inappreciable magnitude as would suffice 

 to generate this energy. 



Before 1896 physicists were just beginning to grasp 

 Maxwell's theories, and to realise more clearly the sim- 

 plification introduced into notions electric and optical by 

 the conception of the ether. The discovery of rays 

 -capable of discharging electrified bodies in air has not 

 only shown the fallacy of our preconceived dogmatic 

 notions as to the division of substances into conductors 

 and dielectrics, but has taught us that the properties of the 

 ■ether are not so simple as we had anticipated. We can 

 only wonder whether Maxwell would have been able to 

 develop his electromagnetic and electro-optic theories 

 had the complications arising from Becquerel and other 

 rays been before him, and the want now makes itself felt 

 •of a second Maxwell, who shall co-ordinate the newly- 

 accumulated mass of experimental facts into the form of 

 a connected mathematical theory. G. H. Bryan. 



NO. 1598, VOL. 62] 



MODERN MICROSCOPES} 



IN spite of the attention which has of late years been 

 paid to the improvements of every detail of microscope 

 construction, it is remarkable how Powell's No. i stand 

 has now existed, practically unchanged, for some fifty 

 years. It may therefore be considered a permanent 

 type, and it is one to which the best modern instruments 

 conform more and more. Its most obvious peculiarity, 

 however, a tripod base, has not yet become general. 

 The heavy horseshoe foot is still in all but universal 

 favour on the Continent, although Powell's base is occa- 

 sionally imitated. Thus the Leitz firm in 1893, and the 

 Hartnack firm in 1898, brought out large model micro- 

 scopes on a tripod base ; Greenough's low-power stereo- 

 scopic binocular microscope (1898) is similarly equipped. 

 This last instrument, which is the most recent binocular 

 novelty, is highly esteemed. It is made by Zeiss, is 

 fitted with porro prisms, and, among other advantages, 

 affords views of the under as well as the upper side of an 

 object. 



English makers have lately paid much attention to the 

 perfecting of cheaper stands with some excellent results. 

 In their new model and educational microscope, Messrs. 

 Ross have reintroduced the principle of a reversing and 

 locking foot, which was first invented by Cuff (ar^a 176S). 

 By this means the instrument acquires great stability 

 when used horizontally. The same firm, in their bac- 

 teriological microscope, use a tripod stand, of which the 

 hind toe is made to fold forward between the two fixed 

 front toes when not in use, thereby economising space 

 in packing. The stage of this, as well as of Baker's 

 microscopes, is fitted with the Nelson horseshoe perfor- 

 ation. The advantage of this device is that in high- 

 power work, when the objective necessarily works very 

 close to the cover glass, the slide can be tilted with the 

 finger, and the focus gradually attained with far less 

 risk to the object than if the slide rested immovably 

 on the stage. 



Messrs. R. and J. Beck's student's microscope and 

 Messrs. W. Watson's " Fram " piicroscope are other ex- 

 amples of really good, small, cheap microscopes. 

 Economy is obtained, not by sacrificing quality of w^ork, 

 but by simplifying the design. Every step in the direc- 

 tion of reducing the cost of a good instrument is too 

 obviously desirable to require demonstration. Some 

 designs strive after cheapness by using a fine adjustment, 

 and trusting to a push-tube motion for the coarse. But 

 if a microscope is to have only one adjustment, most 

 microscopists will prefer a good coarse to an indift'erent 

 fine adjustment. This is the principle of Messrs. Watson's 

 school microscope, which has a coarse adjustment only 

 (diagonal rack and pinion), so good that a ^-inch ob- 

 jective can be accurately focussed with ease. The cost, 

 with eye-piece and objectives, is only three pounds. 



The practical difficulty is, of course, that the great 

 amount of wear upon the coarse adjustment affects in 

 time the evenness of the racking, and produces loose 

 action. But an important piece of progress towards ob- 

 viating this trouble has been made by Mr. E. M. Nelson, 

 who has applied the principle of stepped rackwork 

 (Fig. i). The two similar racks are placed so that 

 their teeth are slightly out of step, the amount of di- 

 vergence being regulated by the upper right-hand screw. 

 The two screws in the centre of the pinion regulate the 

 pressure by which the pinion is forced into the rack. 

 The advantage of the arrangement is not only compen- 

 sation for wear and tear, but rapidity and smoothness 

 of action, for the tube obeys the slightest movement of 

 the milled heads. If experience confirms the favourable 

 opinion with which this novelty has been received, the 

 necessity of a fine adjustment in cheaper stands will 

 disappear. 



1 Fuller accounts of all the irntruments rjferred to will bi found in the 

 Journal oi the Royal Microscopical Society for iSg/, 1S98, 1899 and 1900. 



