206. The glazing of the glass portions is of considerable importance. The brand of glass known as the "best double - thick American" is suitable, though some give preference to heavy ribbed or corrugated "skylight" glass. The glass being chosen, and leakage provided against, the next thing to give attention to, is the management of the light which enters.
In the New England States, what is called a "cone" light is used considerably. A square hole is cut in the roof, and an elevated square cone fitted into it, glazed at the top and sides, varying in height, and the roof slanting both ways. By means of such a construction the light floods into the room in a diffused state, and the operator must work all around the room underneath it, shifting his backgrounds and accessories several times during the day, in order to escape the direct rays of the sun. There is such a thing as getting used to such a light, but I could not be persuaded to work in one unless I had to do it. - Old Agentum.
This is a drawing of a much better light; and, in fact, for some styles of work it is good enough. Still, exactly the same results and effects would have been produced if the light had been laid flat upon the roof and down the side of the building, thereby saving the disfigurement of the house, and also of the room. - E. Z. Webster.
206. In procuring glass for skylights, the photographer who would provide against hailstones and accidents should purchase glass of double thickness, or about one-eighth of an inch thick; although the single, of about one-twelfth, will stand all ordinary usage very well. The second or third quality will answer as well as the first, as the slight imperfections which they contain will not interfere with the passage of the rays of light. In very bright days, I notice that our artists have sometimes an excess of light, which they shut off by means of curtains or screens. But in a very dark day, when they need all the light they can obtain, it is highly necessary that the glass should not be of a dark color. When we speak of the color of colorless glass, we mean that which is perceived in looking at a piece through its edge. The color should be of a light-green or bluish-green, and it should be of a quality which shall not bo deteriorated in the lapse of years by the moisture of the atmosphere, which sometimes produces rust or stain, and sometimes a disintegration of the surface, which gives all the appearance of ground-glass. This defect is occasioned by the two of an excess of alkali in the composition of the glass, and a hurried and insufficient melting of the materials. Another defect, occasioned by impurity of materials or an excess of manganese in the composition of the glass, is the liability to change by sunlight exposure to a yellowish or purplish color, thus diminishing its power to transmit the chemical rays. We are most happy to record that so much has been written concerning these two great defects in glass - making, that they have been almost entirely remedied by glass-makers at borne and abroad. 1 have no doubt that a great source of trouble with some skylights has been, that through a want of cleanliness, the coating of dirt and moisture on the glass has presented as effectual a barrier to the sun's rays as the stain occasioned by rust or the change of color by sunlight. As blue - glass and ground-glass cut off a much larger proportion of the chemical rays than colorless glass, it is not advisable to use them, unless the photographer has so light an apartment that is necessary to exclude some of these rays, or to soften and diffuse the light because of its discomfort to the eyes of the sitters. But it would be much better to accomplish these objects by means of curtains in bright weather, rather than lose the advantage of colorless glass on a very dark day. - Thomas Gaffield.
Considerable has already been said this in Lesson A. First, a likeness is to be secured; next, a round, natural effect, by means of gradation of light and shade. The eyes must not be sunken; the nose and chin must not cast shadows; too glaring a mass of light must not fall upon any one part, and the lights must not cross, and so on.
The slope of the glass roof should be so arranged that at the part where the sitter is placed the light may be incident upon the subject with the least disturbance of its components; an angle of forty-five degrees, if the angular form of roof is used, will be the best, but if the extra expense is not an object, much better results will be obtained from the adaption of the halt-cylindrical form,as the writer's experience of the qualities possessed by two glass studios, in identical aspect and in juxtaposition, but of the two forms in question, leads him to give greatly the preference to the circular, as possessing more evenness and greater rapidity under equal dition of light. It may be accounted for in this manner - owing to the continual variation in the position of the sun, it is not possible to adjust the angle of the straight - sided glass roof in such a manner that the light may pass at right angles, and with the least disturbance or loss of a portion of its power; by the obliquity of its impingement on the glass it will, except at one particular Interval, he incident at an oblique angle to the glared surface, and three - quarter length figure. It should be provided with a semi - opaque screen which may be adjusted over the head of the sitter, and another which may be used at the side to increase the strength of the shadow if necessary. This for a south light is the best arrangement which I have ever seen. - L. G. Bigelow.