432 



NA TURE 



\March 30, 1876 



the potential of the mercury should diminish in the same 

 ratio. Mercury contained in a tube of uniform section 

 would always have the same potential, for as it expanded 

 and lengthened by heat, its specific density would diminish, 

 and the product of density into height would remain 

 constant. If contained in a tube of infinitely small 

 diameter, compared with the diameter of the cup-like 

 extensions, the height would remain constant, whilst the 

 potential would diminish in the ratio of the expansion of 

 mercury, but this is different from the ratio of the diminu- 

 tion of potential of the springs, and in order that these 

 ratios may be accordant, or in other words that the equi- 

 librium of the whole system may be the same at all 

 temperatures, the peculiar form has been employed repre- 

 sented in the illustration, which is between the two forms 

 already referred to. 



The amount of the variation of gravitation with varia- 

 tion of depth is exceedingly small, and requires some 

 method for indicating it. The method employed in the re- 

 sults hitherto tabulated and presented with the paper is by 

 means of electrical contact, which is established whenever 

 a sounding is to be taken, between the centre of the steel 

 diaphi-agm and the end of a micrometer screw, which is 

 at other times insulated from the body of the instrument. 

 The screw is of such pitch, and the circular plate which 

 turns with it has divisions so proportioned, that each 

 division represents a depth of one fathom. 



The readings of the instrument have been compared 

 with soundings taken by means of Sir Wm. Thomson's 

 steel wire sounding apparatus, and the accordance 

 between the two is very satisfactory, especially as the 

 bathometer, from the very nature of its action, gives 

 a mean of the surrounding depths, whilst the sounding- 

 line gives the actual depth below the ship. The reading 

 of the instrument is also effected by means of a spiral 

 glass tube, connecting by means of liquid, less dense than 

 mercury, with the mercury in the upper cup ; this arrange- 

 ment has lately been tried and found to work successfully. 

 The instrument is also available for the measurement of 

 height ; in mountain ascents, however, the elevated land 

 will influence its readings, and allowance would have to 

 be made for the effect of this local attraction. 



The chief disturbing element in the use of this instru- 

 ment is the effect of latitude, which will have to be ascer- 

 tained approximately before its readings can be accepted 

 as true indications of the depth. The difference between 

 the total attraction of the earth at the pole and on the 

 equator amounts to yj^ of its effect at the equator, the 

 rate of increase in travelling from the equator to the pole 

 being as the square of the sine of the latitude. The 

 amount of this variation is easily calculable in fathoms of 

 depth, to be tabulated for use with the instrument. 



The principal value of the bathometer would be to 

 serve the mariner as an additional means of determining 

 his position when he was debarred from taking astrono- 

 mical observation on account of the state of the weather. 

 If the contour of the ocean bed were laid down on charts 

 more perfectly than it is at present, and if these were in 

 the hands of the mariner, he would be able to tell from 

 his bathometer what was the depth of ocean below him, 

 and whether that depth was increasing or decreasing in 

 pursuing his course ; he could also observe the rate of 

 increase or decrease of depth, and in consulting his chart 

 he would be enabled to determine his actual position with 

 considerable accuracy, and thus be forewarned of the ap- 

 proach of danger. 



PHOTOGRAPHY OF THE RED AND ULTRA- 

 RED END OF THE SPECTRUM 



IN 1874 Dr. Vogel communicated to the scientific 

 public that he had been able to photograph the least 

 refrangible rays of the solar spectrum by using what are 

 known as the Uranium Dry Plates prepared by the 



Uranium Dry Plate Company. At the same time he 

 experimented with other bromide plates, using dyes to 

 give them distinctive tints. He then enunciated that the 

 sens itiveness of the plates for the hitherto unphotographed 

 portion of the spectrum was due to the colours employed, 

 and apparently all his efforts have been diverted in elabo- 

 rating this idea. Last summer I commenced a series of 

 similar experiments to see whether the discovery could 

 be made of practical use in photography ; and have 

 arrived at the conclusion, that the colouring matter gives 

 this extended sensitiveness owing to the compound of 

 silver formed and not to the colour itself, in fact, that the tint 

 given to the film necessitates a very prolonged exposure. 

 The additions of resins, nitro-glucose, and other similar 

 compounds containing carbon to the bromized and bromo- 

 iodized collodion soon convinced me that it was the organic 

 salts of silver to which we must look for sensitiveness in 

 the yellow red and ultra-red rays of the spectrum. Nearly 

 every resinous body seems to prolong the photographic 

 spectrum towards the ultra-red, and one or two in par- 

 ticular, also when the film is least colourless when viewed 

 by transmitted light, that then it is probably in the 

 most impressionable condition. Another point which is 

 worthy of notice is, that a film dried from moisture, 

 taking, in fact, the form of a dry plate, is always most 

 readily acted upon by the ultra-red end of the spectrum. 

 Probably this is due to the absorptive qualities possessed 

 by the silver nitrate solution, and not really from an in- 

 creased sensitiveness of the compound salts when dry. 



If ordinary pyroxylin be employed for the collodion it 

 is generally less suitable than if it contain a certain quan- 

 tity of nitro-glucose, or other similar body ; and it fre- 

 quently happens if this be absent entirely that the photo- 

 graphic spectrum will stop short near b. 



Taking a collodion made with pyroxylorin prepared at 

 high temperatures and using the ordinary solvents (in the 

 alcohol of which suitable resin and bromides are dis- 

 solved), it will be found in general that when the silver 

 salts formed through them by the sensitizing bath, or by 

 emulsifying them by direct addition of silver nitrate to the 

 collodion, are presented to the action of the spectrum, 

 the whole of it will be impressed with a developable image. 

 With three prisms of 60° and one of 45° twenty minutes 

 is sufficient exposure to give when the slit is nearly closed, 

 a lens of four feet focus being used as the objective. 



When this is overcome it will be possible (and I hope 

 shortly to do so) to present a complete photographic map 

 of those lines which lie beyond A to a distance at least 

 equal to D — A, a point beyond which I have not as yet 

 been able to obtain an image. The great difficulty to be 

 encountered is that of finding a sharp focus for the dif- 

 ferent points of the invisible spectrum, the change in 

 length from one point to another being very rapid. 



Uranium and iron salts have also furnished me with 

 spectra which are well worthy of notice. With the latter 

 salt more especially the action of the heat-rays is very 

 decided, though at present it seems to me that the ex- 

 posure must be very prolonged, 



I propose at a later date to give details of all the most 

 interesting of these experiments sufficient to enable any- 

 one to repeat them who may desire to do so, 



W. DE W. Abney 



P.S.— It may be as well to state that the best results 

 with resin plates have been obtained when a modification 

 of alkaline development has been adopted. 



RAOUL PICTETS SULPHUROUS ACID ICE- 

 MACHINE 

 THE countries between the 40th degrees of N. and S. 

 latitude have in general too temperate winters to 

 admit of natural ice being obtained in any quantity ; and 

 yet these are the countries in which it is most required. 



