56 



KNOWLEDGE. 



February, 1915. 



adopted. In our own practice we long ago discarded 

 the combined bath, and always resort to separate toning 

 and fixing, making up the toning bath the moment before 

 it is wanted, being careful not to be too sparing of gold, 

 as it is in the use of this in sufficient quantity that the 

 secret of obtaining good tones lies. If, after toning, the 

 prints are fixed in two separate fixing baths, consisting of 

 three ounces of " hypo " dissolved in twenty ounces of 

 water, and afterwards rapidly washed, good prints should 

 be obtained, possessing a reasonable amount of permanence. 



RESTORING FADED SILVER PRINTS.— According 

 to an account by Professor Namias, recently published in 

 the Photographische Korrespondeiiz, faded silver prints may 

 be restored by means of the following treatment. The 

 prints are first bleached in a solution of — 



Copper Sulphate ... ... 5 grains 



Common Salt ... ... 25 ,, 



Water ... ... ... 1 ounce 



The bleached image, consisting of silver chloride, is then 

 well washed, and may be darkened by redevelopment 

 with any of the ordinary developers. The greatest vigour is, 

 however, according to Professor Namias, obtained by the 

 use of sodium stannate. For this purpose a one-per-cent. 

 solution of stannous chloride is taken, and a ten-per-cent. 

 solution of caustic soda added until the precipitate first 

 formed is just redissolved. The print is then placed in this 

 until darkened, after which it is well washed. 



PHYSICS. 



By J. H. Vincent, M.A., D.Sc, A.R.C.Sc. 



FABRY AND PEROT'S INTERFEROMETER.— This 

 instrument is of exceedingly simple construction, and 

 consists essentially of a pair of flat pieces of glass. The glass 

 plates are placed with two surfaces separated from each 

 other by a layer of air, the opposed surfaces being partially 

 silvered. The silvering can be best carried out by cathodic 

 deposition. The surfaces of the plates, which are to be 

 silvered and placed facing each other, must be absolutely 

 true planes, and, when mounted, adjustments must allow- 

 them to be brought accurately parallel to each other. In 

 the interferometer the distance of the glass plates from 

 each other is capable of adjustment, while in another form 

 of the apparatus this distance is fixed. When truly mono- 

 chromatic light passes through the plates, the layers of 

 silver, and the air gap, it does so in a number of ways. One 

 portion goes straight through, another is reflected from the 

 second layer of silver, then from the first layer, and then 

 goes on to join the first portion. The second part of the 

 light has thus been reflected twice ; the first has suffered 

 no reflection. A third portion comes through the plates 

 after four reflections, a fourth after six reflections, and so 

 on. The different portions into which the light is divided 

 will conspire together if the increase in length of path for 

 the successive parts is an exact whole number of wave- 

 lengths, so that, if the plates be viewed with a telescope 

 focused on infinity, a system of circular interference 

 bands will be seen. The bands are due to the obliquity of 

 the paths of all the rays except those striking the plates 

 normally. If now the light is nearly, but not quite, mono- 

 chromatic ; if, for instance, it consists of Ught of two different 

 but closely approximating wave-lengths, two systems of 

 fringes will be formed, one for each kind of light. " Thus the 

 apparatus is a true spectrometer, and can be used to 

 separate the components of spectral lines. 



APPLICATIONS OF FABRY AND PEROT'S INTER- 

 FEROMETER, — The instrument has been used by Fabry 

 and P6rot in the investigation of the intimate structure of 

 spectrum lines. Thus they showed that the green line of 

 thallium w^as triple, the main line having two weak com- 

 panions towards the red end of the spectrum, and measured 

 the differences between the wave-lengths accurately : 



the red cadmium line was proved to be simple. Another 

 application was for the exact comparison of widely differing 

 wave-lengths ; in these experiments the half-silvered 

 plates were separated by as much as 3-2 centimetres. 



THE ETALON INTERFEROMETER —In 1902 Fabry 

 and Perot began to use a modified form of their inter- 

 ference apparatus, which they term an " etalon," or 

 standard. This is derived from the more elaborate form 

 by removing the facilities for altering the distance between 

 the plates, which are held at a fixed distance apart by 

 three pieces of invar. This material is practically inex- 

 pansible by heat, so that, when once the distance separating 

 the plates has been measured, it is capable of being treated 

 as sensibly constant. The fine adjustment for parallelism 

 of the silvered plates is made by springs, which can be 

 pressed down by screws on to the plates over the distance 

 pieces, which, by their compression, enable their effective 

 thickness to be varied. One method of calibrating the 

 6talon is by comparison with the air film of an interfero- 

 meter whose plates can be gradually separated. The 

 measurement of the thickness having been carried out, 

 the 6talon can be used to determine an unknown wave- 

 length. By its means many lines in the spectra of the metals 

 and in the solar spectrum have been accurately measured, 

 these results being now regarded as more reliable than those 

 found by other methods. 



DIATHERMY. — An interesting series of articles on the 

 use of high-frequency electric currents for the production 

 of heat in the body of a patient appears in recent numbers of 

 the Archives of the Rontgen Ray. The author (Dr. Cumber- 

 batch) is in charge of the electrical department at St. Bar- 

 tholomew's Hospital, and thus the information may be 

 regarded as authoritative. The electric current provides 

 a unique method of supplying heat to the deep tissues 

 of the body ; other methods of heating the body act on 

 the skin. When high-frequency current is employed 

 no pain is felt, no muscular contraction is produced, and 

 no sensation other than warmth is perceived, even when the 

 current reaches the root mean square value of from two to 

 three amperes. D'Arsonval showed in 1891 that a current 

 of three amperes could be passed through the human body 

 with impunity, provided that the frequency' of alternation 

 was great. Currents of such strength had not been used 

 previously in electro-therapeutics ; and, as they became 

 better known, it was soon evident that the curative effects 

 wliich followed their use were due to heat ; hence the term 

 " diathermy," to distinguish the method from the older 

 ways of applj'ing high-frequency currents in medicine. 

 The apparatus for the production of the currents used in 

 diathermy consists of two transformers, the first to raise 

 the alternating current from the mains to a few thousand 

 volts. The secondai-y current from this first transformer 

 charges a condenser, wliich is discharged through a spark 

 gap and through the primary coil of the second transformer. 

 The oscillations of the current in this condenser circuit 

 have a frequency of the order of a million a second, and 

 produce in the secondary of the second transformer the 

 current (of the same frequency), which is passed through 

 the patient. The design of the apparatus is such that the 

 current which heats the tissues is, as measured on a hot-wire 

 ammeter, adjustable from zero up to two or three amperes. 

 The spark gap is of a very special construction. It has, 

 in one form of the apparatus, a double gap, each a quarter 

 of a millimetre in length, the spark occurring between the 

 opposed faces of polished silver plates. The sparks take the 

 form of blue films that occupy the air space between the 

 discs. The intervals between the successive trains of high- 

 frequency oscillations are very small, so that the blue film 

 appears to be continuous to the eye ; the discharge is 

 accompanied by a hissing sound. The diathermy current 

 is led to the electrodes, which are in contact with the 

 patient by short, well-insulated, flexible leads, the contacts 

 mide with the body by the electrodes being moistened with 



