I did not remark that the colour of the effluvium was different from one hand to the other; but in reality I did not question much on the subject, as I was most anxious to avoid anything like suggestion. I have never therefore made inquiries upon the possible difference of coloration in the two hands; but I think it would have been pointed out to me, had it been perceived.
Generally the effluvium appears to unite the tips of the fingers of each hand. But it is not always so. Often two or three digital effluvia converge into one of the fingers of the opposite hand, instead of uniting the corresponding fingers.
I noticed that the meteorological conditions and variations of temperature had a decided influence upon the visibility of the effluvia. When the seance-room is very cold, or when the weather is damp or rainy, the effluvia are scarcely perceptible. They appear to reach a maximum intensity in summer, when the temperature is high, and especially when the air is sultry. When the weather is threatening and stormy, the effluvium is thick and clearly visible to me; when the storm has burst, and the atmosphere has cleared, its intensity diminishes.
It often varies according to the individual. Some people give forth an effluvium, which is more visible than that of others. I have not been able to seize any relation between the appearance of the effluvium and the sex, age, and temperament of the various persons with whom I have experimented; on the contrary, a relation seems to exist between the state of health or fatigue and the emission of this mist; it is rarely visible, when the person who emits it is tired or ill.
Such are the principal remarks, which observation of this curious phenomenon has allowed me to make. I have summed them up carefully, but I ought to say, that to me the reality of this appearance does not seem to be demonstrated. After all it may only be due to an effect of contrast. The conditions under which it is observed with the greatest convenience are those, where the hands stand out clearly on a dark background. In drawing the hands away one from the other, the image of the fingers persists perhaps on the retina, and gives rise, maybe, to an illusion; but this explanation is not always sufficient.
There is an optima distance for the realisation of this effluvium. As a rule the effluvium appears denser when the fingers are fairly close together; as they move away the density diminishes; it becomes thinner and more attenuated. But if the hands cease to move, the effluvium disappears. This is the case as long as the tips of the fingers are not more than 2 to 3 centimetres away. If the movement of withdrawal ceases when the finger-tips are within 10 to 15 centimetres proximity, the effluvium remains visible for a longer time. This is what generally happens, but the facts have not always the same regularity. There is, in psychical phenomena, the same diversity and variability, which are observed in other biological phenomena.
I have said that the effluvium persists longer and is best seen when the finger-tips of each hand are within 10 centimetres proximity. Under these conditions, the movement of separation being suspended, the slight mist, which I described, persists several seconds. Sometimes the effluvium is clearly visible, when the fingers are 25 to 30 centimetres apart.
I am inclined to think, that this effluvium is not altogether an imaginary phenomenon. It seems to me to exclude, at least, the hypothesis of the persistence of the retinal image; for the false image does not last so long as the effluvium, under the conditions mentioned by me.
There is yet another explanation. This is that the eye automatically prolongs the clear impression of the fingers on the dark background separating them. This would be analogous to the expansion by irradiation of clear images upon a dark background.
Other reasons, however, make me discard this hypothesis. In the first place, why do some people see the supposed false image vividly coloured and not white? Secondly, if the phenomenon is of retinal origin, why—instead of being thinner, as is the case—does not the image reproduce the form of the finger? Why is it a blue-grey colour and not black, as should be the complimentary image of a finger which appears to be white?