Xov., 1904] 



KNOWLEDGE & SCIENTIFIC NEWS. 



263 



as much need for the yellow screen at sunset as at mid- 

 day." I learn from the coluiims of a contemporary 

 that this passage has " sadly troubled " him and at 

 least two of his readers. This 1 regret, but would 

 suggest that those who wish for further explanations 

 of any matter in these chapters would do well to ad- 

 dress our editor, or me through him, as then lliey will 

 be certain of getting such assistance as I can render. 

 I cannot undertake to read other journals for the sake 

 of tinding comments on what I say here. 



The point of the above extract is that the excessive 

 blue sensitiveness of the plate is an error of the plate, 

 and if not compensated will produce its effect in a 

 photograph taken when the sun is low as well as when 

 the sun is: high. I'hat if the partial loss of blue light 

 as the sun gets low (the light becoming yellowish by 

 reason of this loss) is accepted as compensation in 

 photographing late in the day, ami the yellow screen is 

 discarded, then the colours of the objects photographed 

 will be rendered in the photograph as if they were lit 

 by a midday sun and the yellow screen were used. It 

 would be an anachronism to represent an evening .scene 

 with a low sun as if the quality of the light were such 

 as we get only with a comp.iratively high sun — Ijut we 

 are so used to errors of this kind in photography that 

 our sense of criticism is dulled. The error is in the 

 plate, and whenever the plate is used the screen should 

 be used if it is desired to represent the scene as it 

 appears when photographed. 



Indoors it is possible to use a yellow light instc.id of 

 a screen, and it will be found that ordinary gas or 

 lamp light will compensate the plate error to about the 

 same extent as a yellow screen that requires the ex- 

 posure to be increased four or six times. Rut the 

 photograph taken by gas light will not render colours 

 as they appear by gas light, but as they would appear 

 by a whiter light more similar to daylight. If one has 

 a colour screen that is made to properly compensate 

 the errors of colour sensitiveness of the plate, then the 

 screen should ahoays be used when it is desired to 

 render colours with the tone values as they appear at 

 the time of photographing them. 



T/ie Royal I' holographic Society's Exhibition. — 

 Although the relation between the dates of the opening 

 of this exhibition and the publication of this journal 

 renders it impossible to refer to any of the exhibits 

 until after thej' have ceased to be on view, there arc a 

 few points connected with the scientific and technical 

 section that it may be of interest to refer to. The 

 making of a series of consecutive photographs of 

 changing objects at suitable and st.ited intervals so as 

 to show the character and progress of the change, is 

 an application of photography that has much to com- 

 mend it. There were several examples of such work 

 this year as there have been in previous exhibitions, 

 the most notable being a series of seventy photographs 

 by Mr. W. M. Martin, showing the embryology of a 

 chicken. Of this series some were taken by trans- 

 mitted light, some by a combination of transmitted 

 and reflected lighl, and one by Rontgcn rays ; that is, 

 the illumination was varied in order to best show the 

 required detail — a consideration in such work that (Iocs 

 not always receive the attention that it might. The 

 application of Rontgen ra3's to the demonstration of 

 internal and hidden structures was exemplified in a 

 novel way by Dr. Rodman in a number of radiographs 

 of Mollusca. The internal anatomy of each shell was 

 shown with surprising clearness, and the systems of 

 complex and superposed curves often formed figures of 

 great beauty. 



A portrait taken with ultra-violet radiations obtained 

 by means of the screen devised by Professor R. VV. 

 \\'o(_>(J was shown by .Mr. lidgar .Senior. It merely 

 demonstrates in a striking way the possiljility ol photo- 

 graphing an object quite in the dark. The portrait 

 was very passal)ly focusseil, considering that the ob- 

 ject and the image were both invisible, anti that lenses 

 are not corrected for the purpose of using them in this 

 eccentric manner. A telepholograph of the upper [j.irt 

 of St. Paul's Cathedral, by Mr. .'\. 1"^. .Stiiith, was an 

 excellent production from a technical point of view. 

 I'he magnification was twenty-lour diameters, and as 

 one thousand feet of London atmosphere intervened 

 between the object and the camera, it was an excellent 

 illustration of how an almost hopeless task may l^e 

 siiccessfidly accomplished by selecting suitable ct)ndi- 

 tions. riifjse interested in Mr. J. Iloit Player's in- 

 genious method of copying engra\ings, tVc. (Player- 

 type), will be glad to know that Mr. Player showed 

 some very satisfactory enlargements produced from 

 negatives obtained by his process. 



Coloured I'llins. — The necessity for controlling the 

 character of the light that may be available is ever 

 present with the practical photographer, 'i'oo t)ften he 

 lias to be content with some commercial article made 

 for a e|uite different purpose that happens to be sulfi- 

 ciently near to what he requires to be serviceable. 

 Bookliinders' cloth, coloured tissue paper, hock bottles, 

 and other such makeshifts, are still commonly used, 

 though in a few cases the demand has been met by 

 more suitable and specially-prepared media. A further 

 step in the right direction has recently been made by 

 Dr. G. Krebs, of Offenbach-on-Main, who has put 

 upon the market a considerable assortment of tough 

 coloured films, specially made for the various jjurposes 

 for which coloured media are required. They are 

 known as the " (Jeka llexoid filters." Man\', if not 

 all of them, have been prepared from formula; 

 suggested by Or. Miethe. I have examined several, 

 and can confirm the statement that they do, practically 

 speaking, absorb that [i.-irt of the spectrum that they 

 are stated to in the description of them, ;md that they 

 form such series as are most generally neetled. There 

 are a yellow and three red films for dark-room lamps, 

 transmitting the red and green, the red alone, the red 

 beyond wave-length 610, and the red beyond about C, 

 respectively ; yellow films of three depths for use with 

 orthochromatic plates ; blue, green, ;ind red filters for 

 three-colour work, and many others of various kinds. 

 They are obt.iinable in sheets of all sizes up to 15 by 

 12, and even larger, from the importers, Messrs. A. E. 

 .Stalcy and Co. 



School of Art Wood-Ca.rvirvg. 



Tin. School of .\rt Wood-Carvinj;. South Kensington, which 

 now occupies rooms on the top floor of the new buildiiif,' of 

 tlie Royal School of Art Needlework in Pxhibition Road, has 

 been reopened after the usual sunnncr vacation, and wc are 

 requested to state that some of the free studentships main- 

 tained by means of funds granted to the school by the London 

 County Council are vacant. The day classes of the school 

 are held from 10 to i and 2 to 5 on five days of the week, and 

 from 10 to I on Saturdays. The evening class meets on three 

 evenings a week and on Saturday alternoons. Forms of 

 application for the free studentships and any further parti- 

 culars relating to the school may be obtained from the 

 Manager. 



