To the photographer one of the most important features, requiring his particular attention, is, that he be provided with a good lens. By the remarks given in the preceding pages, he will be enabled, in a measure, to judge of some of the difficulties to which he is _occasionally_ subjected. We have in this country but two or three individuals who are giving their attention to the manufacture of lenses, and their construction is such, that they are quite free from the _spherical_ or _chromatic aberration_. CHAPTER II. 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. [Sidenote: 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. [Illustration: Fig. 13. Fig. 14.] [Sidenote: POSITION OF LENSES IN THE TUBE.]