\20 



NATURE 



[January 5, 191 1 



such a coloured object be examined by means of a spectro- 

 scope, it will be found that a ]X)rtion of the spectrum is 

 partly or completely missing. This missing portion 

 appears as a black band, which is generally known as 

 the absorption band of the colour. If a particular object 

 absorbs most of the constituents from white light, so that 

 only a small portion of the spectrum is transmitted, then 

 that portion may be referred to as the transmission band. 



Since the light which is not absorbed falls upon the 

 eye, the sensation of colour produced is the reverse, or 



W. L. 5000 to 5400 5700 640c 



Photomicrographs of an Eosine stained section. 



complementary, to the colour which is absorbed, so that 

 a light-blue object has an absorption band in the red of 

 the spectrum, a magenta in the green, an orange in the 

 blue-green, and a yellow in the blue-violet. Now this 

 consideration shows that if a colour is to be rendered as 

 black as possible, then it must be viewed or photographed 

 by light which is completely absorbed by the colour, that 

 is, by light of the wave-lengths comprised within its 

 absorption band. 



A useful example is given by the photomicrographs of 



Transmission Spectrum of Wratten B and E Screens. 



a section stained with eosine; this section appears pink, 

 eosine absorbing green and blue-green light from XA. 4700 

 to 5400. If the section is photographed by green light of 

 wave-lengths 5000 to 5400, completely absorbed by 

 eosine, the section is entirely black, the maximum amount 

 of contrast being obtained, and, owing to an excess of 

 contrast, the detail of the section is blocked up. Photo- 

 graphing at X 5700, on the border of an absorption band, 

 a greatly lessened contrast is obtained, which for this 

 particular section will give the best result. If we photo- 



NO. 2149, VOL. 85] 



graph by red light of wave-length 6400, which is com- 

 pletely transmitted by the section, the contrast disappears 

 and the results are f^at and useless. 



This section thus demonstrates the close connection 

 between the colour of the illuminating light and the con- 

 trast produced. A different procedure is required il 

 contrast is to be obtained, not against the background, 

 but within the object itself. 



.\ good case of this is the photography of an unstained 

 section of whalebone ; this is of a yellow colour, and 

 shows ample detail to the eye, but it completely absorbs 

 blue-violet light, and if it is photographed on an ordinary 

 plate sensitive only to blue-violet light, then it shows fa'r 

 too much contrast, appearing as a black detailless mass 

 against the background, and presenting an exaggerated 

 example of the loss of detail which has already been 

 noted in the eosine section photographed by light whicii 

 it completely absorbs. 



The proper procedure in this case is to photograph th. 



'iject by the light which it transmits. The whalebon. 



• ction, for instance, photographed by red light, give-, 



perfectly satisfactory results, showing ample detail in 



tructure. 



The best method of determining the contrast required 

 any object is to examine the object visually under the 

 licroscope first by means of a combination of screens 

 transmitting light absorbed as completely as possible, and 

 then by other screens transmitting light less completeh 

 absorbed, until the degree of contrast obtained is satis- 

 lactory to the eye. 



In the booklet on photomicrography, Messrs. Wrattei; 

 ind Wainwright, Ltd., publish a list 'of the chief micro- 

 scopical stains, giving their absorption bands. By th- 

 help of a special set of screens a section stained with an\ 

 of these colours can be illuminated in such a way as to 

 produce any required degree of contrast. 



The accompanying illustration shows the absorption 

 spectrum of aniline blue, and the transmission band of th ■ 

 filters chosen to produce the maximum degree of contrast. 



In order to estimate exposure, tables are given show- 

 ing the various factors involved, and including a table 

 giving the multiplying factors of the screens used singly 

 and in pairs, the light sources given ranging from th- 

 oil lamp to the open arc. 



PRIZE AWARDS OF THE PARIS ACADEMY 



OF SCIENCES. 

 A T the annual public meeting of the academy, held on 

 December 19, the prizes awarded for the year 

 1910 were announced as follows : — 



Geometry. — The grand prize of the mathematical 

 sciences was not awarded, no memoir having been 

 presented on the stated problem ; Emile Lemoine 

 receives the Francceur prize, and M. Riquier the 

 Poncelet prize. 



Mechanics. — A Montyon prize is awarded to 

 Jules Gaultier, for his inventions in connection 

 witli surveying instruments ; the Fourne3'ron priz- 

 was postponed to 1912. 



Xavigation. — The extraordinary navy prize wr- 

 divided between G. Hilleret (3000 francs), J. L. li 

 Lafrogne (1500 francs), and J. Lecompte (151 

 francs) ; the Plumey prize was not awarded. 



Astronomy. — The Pierre Guzman prize was no' 

 awarded, but the interest accrued was attributed 

 to the late Maurice Loewy, for the whole of his 

 scientific work ; the Lalande prize to P. H. Cowell 

 and A. Crommelin, for their researches in connec- 

 tion with Halley's comet; the Valz prize to 

 St^phane Javelle,' for his work on nebuhns and 

 periodic comets ; the Janssen medal to W. W. Campbell, 

 for his researches in stellar spectroscopy. 



Geography.— The Tchihatchef prize is divided between 

 Dr. Verbeek (2000 francs), for his geological explorations 

 in Borneo, Sumatra, and Java, and Louis Vaillant, for 

 his explorations in Central Asia ; the Gay prize is not 

 awarded, but Carlos Porter receives a mention for his 

 work on the fauna and flora of Chili ; the Binoux prize 

 is divided between Emmanuel de Martonne (1000 francs), 

 for his work in physical geography, A. Bellot (500 francs), 



