December. 1912. 



KNOWLEDCxE. 



477 



additional light action in the green of the spectrum. Follow- 

 ing up this discovery he finally stated that the addition of 

 certain dyes to an emulsion rendered the silver salts sensitive 

 to the less refrangible rays of the spectrum. The value of 

 eosine as a colour sensitizer was discovered by Waterhouse, 

 in 1875, and in 1S79, Ives published his discovery of the 

 orthochromatic etfect obtained by the use of collodion 

 emulsion treated with chlorophyll. In 1883, a patent was 



Figure 503. 



Photograph of a coloured rectangle, taken on an 



ordinary plate. 



obtained by Tailfer and Clayton, for a method of rendering 

 gelatine emulsion orthochromatic by means of eosine together 

 with ammonia, and plates prepared under this patent were 

 placed upon the market by .Messrs. B. J. Edwards & Co., in 

 the following year. Orthochromatic plates, therefore, are 

 those in which a dye or dyes, and sometimes a dye compound 

 with silver, have been added to the sensitive salt, in order to 

 impart sensitiveness towards the brighter colours of the 

 spectrum. The use of such a plate by itself, however, is not 

 found sufficient in practice, as, owing to the still greater 



• Figure 504. 



Photograph of the same colours used in Figure 503. but 



taken on a panchromatic plate. 



sensitiveness of the emulsion to blue and violet light, these 

 colours impress themselves too strongly. In order, therefore, 

 to obtain a correct representation in monochrome of the 

 relative luminosities of the various colours in the subject 

 photographed, a light filter iias to be used to subdue the too 

 active blue and violet rays, as well as to prevent the action of 

 any ultra-violet light. It is also of importance that this light 

 filter should be properly adjusted to the plate intended to be 

 used with it, and that it should not needlessly prolong the 

 exposure. Of late, dj'es have become available which not 



only greatly increase the general sensitiveness of plates treated 

 with them, but especially do so to yellowish light, and to such 

 an extent that it ispossible to obtain pronounced orthochromatic 

 effect with much lighter screens, thereby reducing exposure. 

 The great improvement obtained in the rendering of extreme 

 distances, the variety of gradation in foliage, as well as cloud 

 effects upon the same negative, resulting from the use of these 

 plates and suitable filters, are now generally known, and even 

 when employed without a screen, when working late in the evening 

 or on dull days, they will give better results than an ordinary 

 plate. One thing that should be carefully guarded against is 

 ■■ under-exposure," as its effects appear to be unduly pro- 

 nounced with these plates, when employed with a screen. 

 Perhaps one of the difficulties experienced is when to use a 

 screen. This should always be done when a large amount of 

 blue enters into the colour of the subject, such as blue sky 

 with white clouds, distances, and so on, or in photographing 

 landscapes on a dull day, when the greens will be improved by 

 its use. .Although the introduction of orthochromatic plates 

 was a great step in advance, still when dealing with objects 

 containing much red they fell far short in giving the desired 

 result, owing to the dyes used not imparting sensitiveness to 

 these colours. During the last year or two plates have been 

 placed upon the market which are practically sensitive to the 

 whole of the visible spectrum, and when used in conjunction 

 with a properly adjusted light filter, will render colours 

 very nearly correct as regards their luminosities when translated 

 into monochrome. In the illustrations are seen two examples 

 from negatives taken of rectangular patches of colour. Figure 

 503 being from an ordinary plate, while Figure 504 was 

 photographed upon an llford Panchromatic plate with a 

 screen. The striking difference between the two is at once 

 apparent and serves to illustrate the great advantage of this 

 class of plate in photographing coloured objects, especially if 

 they contain much red. 



ZOOLOGY. 



By Professor J. Arthur Thomson, M..^. 



REPRODUCTIVE PERIODS OF BIRDS.— It has often 

 been pointed out that the reproductive periods in birds are 

 usually well-defined, and that the sex impulse is precisely 

 punctuated. But A. Chapellier has recently called attention 

 (Comptes Rciidus Soc. Biologic, LXXII.) to cases of sexual 

 activity outside the breeding season. He instances the 

 autumnal pairing of wild duck, American swallows, " Blue- 

 birds," Progne subis. Elan us dispar. Strix perlata, and 

 Otiis brachyotus. He asks whether there is not considerable 

 evidence of an autumnal period of sexual e.\citen)ent among 

 birds, — expressed not only in pairing, but in song, attempts at 

 nest-making, and combats. 



VIBRATILE FIK OF THE ROCKLING.— The three- 

 bearded rockling iMotcllit tricirrata) and the five-bearded 

 rockling i.U. mitstcla) are familiar shore fishes — shy, nocturnal, 

 phlegmatic, and non-predaceous. They are fond of lurking 

 under stAes between tide-marks, and feed on crustaceans, 

 annelids, starfish, sea-spiders, and the like. Conspicuous on 

 their back is a modified dorsal fin, consisting of a series of 

 small processes, which are almost continuously in rapid 

 vibration, and, anterior to these, a ray which is much longer 

 and thicker than the others, and has nuich less power of 

 movement. .Around the base of the rays there is a groove, 

 and bordering this the skin is kept clear and clean. It has 

 been suggested that this vibratile fin. whose movement is 

 conspicuous (observable from three to six feet in the still 

 water of an aquarium), may serve as some sort of " lure "; but 

 Dr. J. Stuart Thomson brings forward strong evidence pointing 

 in another direction — to the use of the vibratile fin, or the area 

 immediately around it. as a taste-organ or food-locating organ. 

 The skin around the groove proves to be very rich in taste- 

 buds, and experiments show that this area is extremely 

 sensitive to the proximity of food. The vibration keeps the 

 " receptor " area clean, and brings to it currents with theis 

 subtle indications of available food material. 



