M. Chopin's letter is divided into two parts. In the first two pages the author explains how, under certain conditions, different from those indicated by M. Lajoie, plates may give direct positives ; for example, the following process will give this result : " After an ordinary light development wash the plate, .cover it with a silver solution having an alkaline reaction, or any weak alkaline solution, and recommence to develop." Naturally there can be no question of M. Pia having exercised any such manipulation; but M. Chopin explains that the phenomenon may happen accidentally, and had indeed occurred to himself " in the preparation of special negatives for the reproduction of Limoges enamels/' M. Chopin has then the right to use the following phrase : " There is reason to think that when this phenomenon is accidentally produced, one should seek for the cause either in a want of acidity in the silver solution or in the in-sufficent alkalinity of the water used for washing the plates." I do not guarantee that the terms used by M. Chopin are quite exact, and that the washing of a plate in water not sufficiently alkaline would altogether alter its character, but this is what M. Chopin says. In certain conditions it is possible that a plate may become positive without having been over-exposed.
But we will not labour the point. Such exceptional conditions were not present, since M. Pia's plate is really a negative. This point we may consider certain.
We now come to a more subtle objection, which is that although a plate may be generally negative, certain parts of it may not be perfectly so, owing to the effect of colour—yellow, for instance, often comes out black. We do not quite see how this affects our consideration of the Holy Shroud. The argument is only tenable if parts of the object are many coloured, which is not the case here.
We consider then that we have established our first position, which is that the impressions on the Holy Shroud are really and effectively shown there in negative.
If the impressions on the Shroud have the character of a negative, if they are shown with the real values reversed, we cannot believe that they have been painted thus. Of two alternatives one must be taken as true: either they have been executed positively, like all paintings, and have been transformed by time into the negatives which we see, or else they are not paintings at all. Before discussing these alternatives let us see if the impressions are such as to preclude altogether the hypothesis of an ordinary painting done directly in negative fashion.
Let us quote once more from M. Chopin's letter : " M. Loth asserts that the Holy Shroud is not a painting, and that no artist, however clever, could have produced such a work in negative. As for this assertion I am of a contrary opinion, for I am fully persuaded that such crude work could easily have been produced by an artist; but to paint in this manner would be so absurd and incomprehensible that I hesitate to assert that it was so done." Now this quotation contains two propositions, both of which we must oppose. It is not absurd for a painter to reproduce a work in negative, but if he had tried to do so he would inevitably have failed ; practically, then, we agree with M. Chopin, and will refute the hypothesis of a direct negative painting, but for different reasons than those given by him.
To begin with, with regard to a presumed painter, we will grant M. Chopin much more than he asks : we will grant that a clever artist might have painted these impressions in negative, although it scarcely seems possible to do so ; the very term " negative " has only had a meaning since the discovery of photography. The men of the fourteenth century had no reason to suspect the possibility of the inversions of lights and shades which are produced by the action of light on a sensitive plate ; but was not this much knowledge indispensable before it would occur to any one to reverse on a cloth the normal position of lights and shadows ?
But what is the point at issue ? Is it a question of the painting of a portrait, or that a certain piece of linen has been the winding-sheet of Christ ? To intelligent observers all that Christ could have left on His Shroud would be the print of His limbs and the traces of His blood, and not, properly speaking, a portrait of any sort. Now an imprint which marks the raised parts more than the hollows is in itself a negative.
It may be said that I am making things too easy for the forgers of the Middle Ages, who would not have reasoned in such subtle fashion ; that the Holy Shroud would be even more worthy of veneration had the portrait of the Saviour been clearly visible to any observer.
Well, that would simplify the question, for there would then be no hesitation in admitting that the Holy Shroud had not been painted in negative. And yet we are bound in loyalty to science to continue our argument. The arrangement of the pictures alone is enough to show us that if they really are the work of a painter, this painter has endeavoured to give them the effect of being impressions. If he were ingenious enough to know that the two bodies should lie head to head he must have also known that the impressions give us an inverted mould. To ascertain this he need only have placed his blackened hand upon a white wall, or smeared his face with red ochre and then covered it with a cloth.
In either case the marks left on the linen or on the wall could only have been caused by such portions as would appear in relief if the light shone full upon them. The half-tints and shadows on the contrary would leave fainter marks, or no marks at all. This is what fraud might have tried to reproduce in a painting.
Such an attempt must have been faulty in its execution. The real question therefore is, Are the pictures such as an artist of the fourteenth century could have executed if he had tried to imitate in his painting a true impression ? M. Chopin gives a decided " yes " to this question, but our own opinion is different.