PHOTOGRAPHIC CORRESPONDENCE.
Photographic Exhibition.—We understand that the Photographic Society has made arrangements for an exhibition of photographs in the metropolis during the months of January and February next. The exhibition will not be confined to the works of native photographers, but will comprise specimens of the most eminent foreign artists, who have been specially invited to contribute. From the advances which have been made in this favourite art, even since the recent exhibition in the rooms of the Society of Arts, we may confidently anticipate that the display on the present occasion will be one of the highest interest.
How much Light is obstructed by a Lens?—Can any of your scientific correspondents furnish me with an approximation to the quantity of light which is transmitted through an ordinary double achromatic lens, say of Ross, Voightlander, or any other celebrated maker?
Lux.
Stereoscopic Articles.—I cannot agree to my opponent's assumed amendment (?) (Vol. viii., p. 419.) space, for the simple reason that it would be virtually abandoning the whole of the points in dispute between us; when farther discussion and more mature consideration, only tend to convince me more firmly of the correctness of the propositions I have advocated, viz.:
1st. That circumstances may and do arise in which a better result is obtained in producing stereographs, when the chord of the angle of generation is more or less than 2½ inches.
2nd. That the positions of the camera should not be parallel but radial.
I certainly thought that I had, as I intended, expressed the fact that I treat the cameras precisely as two eyes, and moreover I still contend that they should be so treated; my object being to present to each eye exactly such a picture and in such a direction as would be presented under certain circumstances. The plane of delineation being a flat, instead of a curved surface, has nothing whatever to do with this point, because the curves of the retinas are not portions of one curve having a common centre, but each having its own centre in the axis of the pupil. That a plane surface for receiving the image is not so good as a spherical one would be, is not disputed; but this observation applies to photographs universally, and is only put up with as the lesser of two evils. A plane surface necessarily contracts the field of view to such a space as could be cut out of the periphery of a hollow sphere, the versed sine of which bears but a small ratio to its chord.
There is another misunderstanding into which my opponent has fallen, viz. the part of the object to be delineated, which should form the centre of radiation, is not the most contiguous visible point, but the most remote principal point of observation. I perceive that this is the case from two illustrations he was kind enough to forward me, being stereographs of a
To push an argument to the extreme to test its value, is quite right; but this goes far beyond the extreme, if I may be allowed such a very Hibernian expression.
No object, however minute, can be clearly seen if brought nearer to the eyes than a certain point, because it will be what is technically called out of focus. It is true that this point differs in different individuals, but the average distance of healthy vision is 10 inches. Now, adopting Mr. Merritt's own standard of 2½ inches between the eyes, it is clear that supposing the central point had been rightly selected, the distance between the cameras was only double what might have been taken an extreme distance. It is scarcely necessary to suggest what a person devoid of taste (in which category I am no doubt included) might do in producing monstrosities by adopting the radial method, as such an one is not very likely to produce good results at all.
I now address myself to another accusation. It is quite true that I am unacquainted with the scholastic dogmas of perspective, but equally true that I am familiar with the facts thereof, as any one must be who has studied optical and geometrical science generally; and while I concur in the propositions as enunciated for a one-eyed picture, I by no means agree to the assumption that the "vanishing points," in the two stereographs taken radially with the necessary precautions, "would be so far apart, that they could not in the stereoscope flow into one;" on the contrary, direct experiment shows me, what reason also suggests, that they do flow into one as completely as in nature when viewed by both eyes.
I put the proposition thus, because I do not hesitate to avow that in nature, as interpreted by binocular vision, these points do not absolutely, but only approximately, flow into one; otherwise one eye would be as effective as two.
I have not the smallest objection to my views being considered "false to art," as, alas! her fidelity to nature is by no means beyond suspicion.
Lastly, as to the model-like appearance of stereographs taken at a large angle, for the fact I need only refer the objector to most of the beautiful foreign views now so abundant in our opticians' shops: for the reason, is it not palpable that increasing the width of the eyes is analogous to decreasing the size of the object? and if naturally we cannot "perceive at one view three sides of a cake, two heads of a drum, nor any other like absurdity," it is only because we do not use objects sufficiently small to permit us to do so. Even while I am writing this, I have before me a small rectangular inkholder about 1¼ inches square, and distant from my eyes about one foot, in which the very absurd phenomenon complained of does exist, the front, top, and both sides being perfectly visible at once: and being one of those obstinate fellows who will persist in judging personally from experience if possible, I fear I shall be found incorrigible on the points on which your correspondent has so kindly endeavoured to enlighten me.
Geo. Shadbolt.
To introduce Clouds (Vol. viii., p. 451.) as desired by your correspondent Σ., the negative must be treated in the sky by solution of cyanide of potassium laid on in the form desired with a camel's hair pencil. This discharges a portion of the reduced silver, and allows the light to penetrate; but great care is required to stop the action by well washing in water before the process has gone too far. White clouds are produced by painting them in with a black pigment mixed in size.
Geo. Shadbolt.