Читать книгу A Practical Manual of the Collodion Process - S. D. Humphrey - Страница 4

Table of Contents

CAMERA—ARRANGEMENT OF LENSES—CAMERA TUBES—CAMERA-BOXES, BELLOWS, AND COPYING—CAMERA STANDS—HEAD RESTS—CLEANING VICE—NITRATE BATH—LEVELING STANDS—PRINTING FRAMES—COLLODION VIALS.

Babtista Porta, when he saw for the first time, on the walls of his dark chamber, the images of external nature, pictured by a sunbeam which found its way through only a small hole, little thought of the importance which would be attached to the instrument he was, from this cause, led to invent. The camera obscura of this Italian philosopher remained as a mere scientific toy for years, and it was not until Daguerre's discovery that its true value was estimated. It now plays a very important part in giving employment to at least ten thousand persons in this country alone.

It is of the utmost importance, in selecting a set of apparatus, to secure a good camera; for without such no one can obtain fine pictures. In testing it, see that it gives the pupil of the eye and lineaments of the features sharp and distinct; and that the whole image on the ground glass has a fine pearly hue. Look also to the field, and observe that the focus is good at the centre and extreme edges of the ground glass, at the same time. A poor camera generally gives a misty image, with the lights and shades apparently running together. The best American cameras are fully equal to those imported, while they cost much less; but there are great numbers sold which are not worth using.

CAMERA TUBES AND LENSES.

If a lens gives a well defined image on the ground glass, it should do the same on the plate. Many a valuable lens has been condemned for failing in this, merely in consequence of the plate-holder not being in focus with the ground-glass. In case of deficiency in this, put a glass into the holder, lay a rule across the face, and measure the distance between them very exactly; measure the ground-glass in the same way, and make the distance agree perfectly, by moving the ground-glass either back or forward in the frame, as the case may be, so that the surface of the glass plate shall occupy precisely the same position as the face of the ground-glass when in the camera.

Fig. 13.	Fig. 14.

POSITION OF LENSES IN THE TUBE.

It is very desirable that the operator should understand the arrangement of the lenses in the tube; it not unfrequently happens, that in taking out the "glasses" to clean them, he does not return them to their proper places, and the result is that his "camera is spoiled." A couple of illustrations and a few remarks will be sufficient to enable any one to replace the lenses in them properly. Fig. 13 represents the tube for holding the lens, and Fig. 14 shows their arrangement. It will be seen that the two back lenses have a small space between them; this separation is kept by a small tube or ring of the same circumference as the lens. The two front lenses are nearest together. It will be observed that the two thick lenses are towards each other; these are made of flint glass containing much oxide of lead. The other two are double convex, and are made of crown glass. By noting the fact that the two cemented lenses go in the front of the tube, the glass having the thickest edge goes inside, and that the thickest lens of the other two goes in first, from the back of the tube, it will not be easy for the operator to make a mistake in returning the "glasses."

"I will remark that a diaphragm diminishes both chromatic and spherical aberration, by cutting off the outside portion of the lens. It lessens the brilliancy of the image, but improves the distinctness by preventing various rays from interfering with and confusing each other; it also causes a variety of objects at different distances to be in focus at the same time."