4i6 



NA rURE 



[March i, 1900 



squared paper on which the divisions are thirds of the unit, so 

 that a boy will have it fixed in his mind that 3^ is exactly 

 logaio. 



My intention was to give a good exercise on the use of squared 

 paper for interpolation ; it happens to be a method of calculat- 

 ing logarithms correctly to as many places as we please. It 

 keeps before a boy the simple notion that a common logarithm 

 is the index of a power of lo. The idea before Mr. Dufton's 

 pupil is much more complex, and he is not likely to find the 

 logarithm of any number correct even to the second figure. 



February 19. John Perry. 



RECENT PROGRESS IN PHOTOGRAPHY. 



THE progress of knowledge and of skill in its applica- 

 tion is often so gradual that although there may 

 be a vast difference between the condition of affairs a 

 few years ago and at the present time, it may be im- 

 possible to discover definite steps in the general for- 

 ward movement. And after a few specific cases of 

 undoubted progress have been singled out, it may be 

 that they, or some of them, will subsequently prove to 

 be only side issues of comparatively trivial importance. 

 On the other hand, a circumstance that is quite obvious 

 to the scientific student may be worked upon by a com- 

 mercial firm and so advertised that a method of work 

 becomes largely modified, and a practical advance 

 effected without any addition to our knowledge. 



It has been known from the earliest of photographic 

 times that sensitive surfaces require a certain minimum 

 of exposure before any change in them can be detected. 

 But the abolition of so-called dark rooms for printing by 

 development, and the employment of the same light for 

 development as for exposure, was a possibility almost 

 neglected until the American " Velox " paper was intro- 

 duced. Of course, for exposure the printing-frame is 

 held very near to the light, and during development the 

 print is shaded. This method of work may be only a 

 passing fashion, but it has revolutionised the printing 

 arrangements of many an amateur, and been so ap- 

 preciated that several firms have put slow papers and 

 lantern plates on the market for working in the same 

 manner, and other makers are seriously discussing 

 whether it is desirable to do so. 



A modification of pigment printing that has recently 

 been introduced by Mr. Thomas Manly constitutes a 

 more substantial advance, though whether it will be so 

 widely appreciated as the silver papers slow enough for 

 development in gaslight remains to be seen. He calls 

 his process " Ozotype." So far as concerns important 

 and serious work, a small improvement in either carbon 

 or platinum printing is likely to be of more real value 

 than any possible change in silver printing. But Mr. 

 Manly's is not a mere modification of detail. Putting 

 it simply, he exposes the transfer paper instead of the 

 pigmented tissue under the negative, and thus by one 

 transfer he gets a print that is not laterally inverted, a 

 result which requires a double transfer by the ordinary 

 method, or else the making of a special inverted nega- 

 tive for single transfer. As the paper exposed is not 

 pigmented, the progress of printing can be judged of by 

 inspection, and there are other advantages of a less 

 important character, such as the absence of any 

 need for "safe edges" to the negatives. The paper 

 that is exposed under the negative is made sensitive by 

 means of a mixture of potassium dichromate and a 

 manganous salt. On exposure, the chromate is reduced 

 and the manganese is thereby oxidised, and both pro- 

 ducts of the change are insoluble in water. By washing, 

 an image that has oxidising powers is obtained, and this 

 may be utilised in many ways. It does not seem to 

 deteriorate by ^eping. To pigment the image, a piece 

 of carbon tissue is soaked in a weak solution containing 

 acetic acid, hydroquinone, and ferrous sulphate, squeegeed 



NO. 1583. VOL. 61] 



on to the print and allowed to dry. Development is 

 effected as usual in carbon printing. Mr. Manly sup- 

 poses that the acetic acid causes the manganic com- 

 pound to reoxidise the chromic compound to a soluble 

 salt (chromate?) which is absorbed by the gelatine of 

 the tissue. In the gelatine the hydroquinone reduces 

 the chromium compound to the chromic condition, 

 which, as usual, renders the gelatine insoluble. This 

 appears to be the inventor's working theory. Final 

 practical details are not yet published, but it is certain 

 that the process is capable of yielding good and useful 

 results. 



The toning of prints in which the image consists of 

 metallic silver by causing it'to act on the solution of the 

 ferricyanide of a metal, reducing it, and thereby causing 

 the deposition of the corresponding ferrocyanide on the 

 image, is a method that has long been known. The ferro- 

 cyanide of copper, being of a reddish-brown colour, is a 

 desirable toning material, but copper ferricyanide is not 

 soluble in water, and the various solvents hitherto em- 

 ployed have. not given satisfaction. Mr. W. B. Ferguson 

 has just shown that a solution of potassium citrate is a 

 solvent for the ferricyanide that serves perfectly, and has 

 thus converted an almost useless process into one that is 

 easy and certain. The colour appears to be rather 

 browner than that yielded by the corresponding uranium 

 process, and it is hoped that the colouring matter will 

 prove more permanent. 



There have recently been quite a number of new intro- 

 ductions that affect the production of negatives. Fore- 

 most among these stand the rapid " spectrum plates " of 

 Cadett and Neall, and the colour screens adjusted thereto 

 by Mr. Sanger Shepherd. Red sensitive plates have 

 been made before, but the even sensitiveness throughout 

 the spectrum that these plates show, has never, in our 

 experience, been equalled. But no plate will of itself 

 render the various colours according to their proportional 

 visual brightness. The sensitiveness to blue and violet 

 vastly preponderates in all cases, and in order to reduce 

 this light and so compensate for the excessive sensitive- 

 ness thereto, various coloured screens have been in use. 

 For general purposes, this screening has been done by 

 the roughest of methods, whether yellow glass or dyed 

 films have been used. But Mr. Sanger Shepherd has 

 prepared screens that are adjusted to the plates within a 

 small margin of error, using for the testing the method of 

 colour sensitometry introduced by Sir William Abney. 

 With such an adjusted colour screen and plate a coloured 

 object can be photographed in almost any light so that 

 the print will give the correct comparative luminosities 

 of all the colours as they appear under the conditions 

 when photographed. The chief exception is that light of 

 less refrangibility than the solar line C, or about that, is 

 not represented, this small portion of the less refrangible 

 red being reserved as a light useful when making the 

 plates and working with them. Colour screens are pro- 

 vided for dark-room lights, that transmit only the least 

 refrangible red, far enough from where the practical seti- 

 sitiveness of the plates begin to furnish a light that is 

 quite unable to harm the plates under the usual con- 

 ditions of development, &c. Mr. Sanger Shepherd also 

 prepares colour screens for spectrum plates adapted for 

 the three-colour process of reproduction, and as they are 

 adjusted by measurement and to the same plate, this is a 

 considerable step towards the simplification of work and 

 the ensuring of correct results. 



Some new intensifiers for negatives have recently been 

 suggested, but as their effects have not been thoroughly 

 investigated, they should not replace the use of mercuric 

 chloride followed by ferrous oxalate, which is the only 

 method that has yet been shown to give a definite result 

 equally proportioned over the whole negative. Arn- 

 monium persulphate, as a reducer, has been shown to thin 

 the image to a nearly equally proportioned degree all 



