126 



SCIENTIFIC NE^VS. 



[Feb. lo, 1 8 



lines, it is necessary first of all to ascertain the normal 

 requirements of the particular brand of plates we may 

 happen to be using. To get at this the manufacturer's 

 formula will have to be analysed. Ascertain how many 

 grains of pyro, how many grains of bromide, and how 

 many minims of ammonia are recommended to each 

 ounce of water. This will be a standard to work by, 

 though of itself not necessarily infallible, and by no means 

 to be trusted implicitly. 



Exposure and subject next claim attention before 

 mixing up the developer. 



Is the particular plate we are about to develop under 

 or over-exposed ? If under-exposed the developer must 

 be weak in pyro and strong in ammonia. If, on the 

 other hand, it has been over-exposed, we shall require 

 more pyro and very little alkali. 



In order to learn whether the plate has been 

 correctly exposed or otherwise, it is necessary to submit 

 it first to a tentative development. 



The subject has next to be considered. Are there 

 strong contrasts in the subject ? If so the developer must 

 be weak in pyro and strong in ammonia. If but little or 

 no contrast, the reverse of this. 



Undoubtedly slow development is the secret of success. 

 We may develop slowly in two ways, either by using a 

 large proportion of restrainer or a small proportion of 

 alkali. Under no circumstances whatever should the 

 full strength of alkali be given at once, but by judicious 

 increments. 



Some few amateurs object to the slow method of 

 development on the grounds of its shghtly discolouring 

 the film. The "clearing solution," however, will soon 

 remedy this defect. 



Many amateurs fail by using too small a quantity of 

 solution. This is false economy. In order to mix up a 

 developer in accordance with the dictates of our judg- 

 ment, it is absolutely necessary to keep our chemicals in 

 separate bottles. Ten per cent, solution is by far the 

 best method. Bromide will keep good indefinitely in 

 solution. Not so with ammonia. If the pyro be made 

 up in solution a preservative chemical will have to be 

 added. Sulphite of soda is a great favourite with many 

 for this purpose. To obtain it absolutely pure, however, 

 is a difficult task. Commercial sulphite of soda is largely 

 adulterated, and even the purest soon oxidises. There 

 are two more objections to its use. The first one is, to 

 be effective it is necessary to use it in very large quan- 

 tities, in the proportions of four of sulphite to one of 

 pyro ; and, in the next place, it is alkaline and needs 

 neutralisation. 



Meta-bisulphite of soda is free, however, from all these 

 objections, and its use to many amateurs seems entirely 

 unknown. It is used weight for weight with pyro, 

 preserves the solution effectively, is not liable to oxidisa- 

 tion, is of itself acid, and therefore requires no citric or 

 other acid to remove its alkaline reaction. The resulting 

 negative, too, is beautifully clear and remarkably free 

 from stains. 



One more question still remains to be answered — How 

 far shall the density be pushed ? Is it better to under 

 or over-develope ? The least evil of the two is certainly 

 the latter, for a negative can be reduced, but development, 

 once ceased, no further detail can be added. 



The proper standard density can only be learnt by 

 experience. It should consist of fine gradations. All 

 desirable detail should be developed up to printing 

 strength, and no further. 



THE MUTUAL RELATIONS OF THE 

 SENSES. 



TT has been commonly supposed that if one or more of 

 the senses of a man is wanting, or is temporarily in 

 an inactive condition, the others, as if by way of com- 

 pensation, become increasingly active, and assume a part 

 of the duties of those that are missing. Thus we frequently 

 hear how the blind acquire such a delicacy of touch and 

 hearing that through them they can derive much of the 

 information which normal persons obtain through the 

 medium of eyesight. Nor can we wonder that such 

 should be the case. Without inquiring whether the 

 auditory organs of a blind man receive an unusual 

 amount of nervous energy, we can very well understand 

 that he will attend closely to the sound-waves which 

 reach his ears, and by careful comparison he can form a 

 fairly accurate notion of the position and the distance of 

 solid bodies, such as objects of furniture in a room, or 

 walls and houses in a street. 



But we find also cases where the organs of one sense 

 seem to assist those of another in the discharge of their 

 respective functions. Thus it is a well-known fact that 

 the most experienced wine-tasters find themselves quite 

 astray if they attempt to distinguish between different 

 vintages in darkness. Certain experiments on this 

 mutual action of the sense-organs have been made by Dr. 

 Urbanschitsch, of Vienna, and the results which he has 

 reached are most curious. Sensations of hearing for the 

 time being render the sight more acute. Thus the ex- 

 perimentalist placed coloured plates at such a distance 

 that the colours could scarcely be distinguished. If various 

 sounds were then produced, the colours of the plates 

 were recognised more distinctly the higher the pitch of 

 the sounds. Or again, printed matter could be read 

 more clearly in a sparingly illuminated room or at an 

 unusual distance if the ears were simultaneously taking in 

 sounds than in complete silence. In like manner the 

 activity of the eyes, or at least their exposure to light, 

 aids us in the recognition of sounds. Thus the ticking 

 of a watch is more easily heard in the light than in the 

 dark, and if the eyes are open than if they are closed. 

 The various colours of light vary in their effects upon 

 our organs of hearing. Red and green lights seem 

 to strengthen our perceptions of sounds, but blue and 

 yellow lights weaken them. Some musicians, however, 

 to whose judgment Urbanschitsch referred the question, 

 were of opinion that not only red and green, but yellow 

 and blue light intensified sounds by about one-eighth, 

 but violet had a contrary effect. 



Corresponding effects were observed in the case of 

 taste and smell. Light in general, and in particular red 

 and green light, heighten their sensitiveness, whilst blue 

 and yellow light, and still more darkness, reduce it. If a 

 person is exposed to red or green light he can taste, not 

 merely with the anterior edges of the tongue, but with 

 its whole surface. Smell and taste reciprocally inten- 

 sify the perceptions of sounds and sights. There is a 

 curious reciprocal action between the sense of touch and 

 the heat sense, which though they have not distinct 

 organs are certainly not identical. If the skin is tickled 

 with a hair, and the hand is then plunged into hot water, 

 the sensation ceases. If, on the other hand, any part of 

 the body is tickled, and the hand or foot is thrust into 

 cold water the chilly feeling is intensified. 



These experiments need to be extended and con- 

 firmed, which may be done the more easily as there is no 

 need of any dehcate or expensive apparatus. 



