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s to 3 inches diameter； and from 8 to 12 inches focal length will answer much better。 For single lenses； the aperture in front should be placed at a distance from it； corresponding to the diameter； and of a size not more than one third of the same。 A variety of movable diaphrams or caps； to cover the aperture in front； are very useful； as the intensity of the light may be modified by them and more or less distinctness and clearness of delineation obtained。 These caps alway come with Voitlander instruments and should be secured by the purchaser。
Though the single acromatic lens answers very well for copying engravings； taking views from nature or art； for portraits the double should always be used。 The extensive manufacture of the most approved cameras； both in Europe and in this country； obviates all necessity for any one attempting to construct one for their own use。 Lenses are now made so perfect by some artisans that； what is called the 〃quick working camera〃 will take a picture in one second； while the ordinary cameras require from eight to sixty。
The camera in most general use is that manufactured by Voitlander and Son of Germany。 Their small size consists of two seperate acromatic lenses； the first； or external one； has a free aperture of 1 1/2 inches； the second； or internal； 1 5/8 inches； and both have the same focus； viz： 5 3/4 inches。 The larger size differs from the smaller。 The inner lens is an achromatic 3 1/4 inches diameter； its focal length being 30 inches。 The outer lens is a meniscusthat is bounded by a concave and convex spherical surface which meethaving a focal length of 18 inches。 For every distant view； the aperture in front is contracted by a diaphram to 1/8 of an inch。 By this means the light is reflected with considerable intensity and the clearness and correctness of the pictures are truly surprising。
THE AMERICA instruments are constructed on the same principle and many of them are equally perfect。 Mr。 Edward Anthony of 205 Broadway； New York city； has constructed； and sold cameras fully equal to the German and for which Voitlander instruments have been refused in exchange by the purchaser。
The ordinary camera box （see fig。 5； a） varies in size to suit the tube； and is termed medium； half； or whole。 Within the box is a slide to assist in regulating the focus； and in enlarging or diminishing the picture。 In one end of this slide is a springed groove into which the ground…glass spectrum （g fig。 5） is slid； for the purpose of more conveniently arranging the focus。 After the plate is prepared it is placed in the holder partly seen at e； fig。 5； and covered with the dark slide f； fig。 5； the spectrum is then withdrawn and the holder takes its place； and the lids d； d； are closed after removing the dark slide f。 The plate is now ready to receive the image； and the cap c may be removed to admit the light into the box。
A camera constructed by Voitlander is thus described by Mr。 Fisher。 〃It is made entirely of brass； so that variations of climate has no effect upon it。 It is very portable and when packed in its box； with all the necessary apparatus and materials for practising the Daguerreotype art； occupies but very little space。 It is not； however； well adapted for the Calotype process。〃
〃The brass foot A （fig。 7。）； is placed on a table； or other firm support； and the pillar B。 screwed into it； the body of the camera； C； C is laid into the double forked bearing D。 D。 The instrument is now properly adjusted by means of the set screws； e； e； e； in the brass foot； or it may be raised； lowered； or moved； by the telescope stand； and when correct； fixed by the screw b。 The landscape to be delineated is viewed either through the ＇hipho_7。gif＇ small lens； g； or with the naked eye on the ground glass plate H； the focus being adjusted by the screw I。 The optical part of the instrument consist of the small set of achromatic lenses already described。 When the portrait or view is deleniated on the ground glass to the entire satisfaction of the operator； the brass cap L is placed over the lens； and the entire body is removed away into the dark； taking care not to disturb the position of the stand。 The body is now detached at the part H； and the prepared paper or plate enclosed in the brass frame work introduced in its place； the whole is again placed upon the pedestal； the brass cap L is removed； by which the paper or plate is exposed to the full influence of the light； after which the cap is again replaced。
Mr。 Woodbridge； of this city； has constructed an instrument for taking full length portraits on plates 10 by 13 inches； which is worthy of some notice。 It is a double camera； consisting of two boxes； placed in a frame； one above the other； and so arranged as to slide easily up and down。 After the focus has been adjusted； on the object； in both cameras； the plate is put into the upper box； in the manner already described； until the superior portion of the figure is complete； it is then placed in the second box and the lower extremities obtained。 The adjustment of the instrument is so complete that ＇hipho_8。gif＇ a perfect union of the parts is effected in the picture without the least possible line of demarkation being visible。 Fig。 8 gives a front view of this instrument。
Fig。 9 represents Talbot's Calotype Camera；a very beautiful instrument。
The copying camera box has an extra slide in the back end； by which it may be considerably lengthened at pleasure。
II。CAMERA STAND。The best constructed stands are made of maple or blackwallnut wood； having a cast iron socket （a； fig。 12；） through which the sliding rod b passes； and into which the legs c； c； with iron screw ferules are inserted。 The platform d is made of two pieces； hinged together； as at e； and having a thumb screw for the purpose of elevating or depressing the instrument。 ＇hipho_9。gif＇
III。 MERCURY BATH。Fig。 13 gives a front view of the mercury bath now in general use in this country for mercurializing and bringing out the picture。 It is quite an improvement on those first used。 To make it more portable it is in three pieces； a b and c； having a groove e on one side to receive the thermometre tube and scale by which the proper degree of heating the mercury is ascertained。 Into the top are nicely fitted two or three iron frames； with shoulders； for the plate to rest in； suitable for the different sizes of plates。 The bath is heated by means of a spirit lamp placed under it。 From two to four ounces of highly purified mercury are put into the bath at a time。
IV。 PLATE BLOCKS AND VICES。There are several kinds of this article in use； I shall describe the two best only。
Fig。 10 gives an idea of the improvement on the English hand block。 The top a is perfectly flat ＇hipho_10。gif＇ and smootha little smaller than the plate； so as to permit the latter to project a very little all aroundhaving at opposite angles c c two clasps； one fixed the other moveable； but capable of being fastened by the thumb screw d； so as to secure the plate tightly upon the block。 This block turns upon a swivle； b； which is attached to the table by the screw c； This block is only used for holding the plate while undergoing the first operatio