THE AMEEICAN MONTHLY 



[January, 



gent rays coming from an ocular will 

 project an image upon a plain sur- 

 face ^vith the same accuracy as the 

 rays coming directly from the ob- 

 jective. The ocular, it would seem, 

 inevitably must inti-oduce aberrations 

 of its own, which, \vhile they might 

 not be noticeable in a small field, 

 would become very conspicuous over 

 a large plate. From considerations 

 of this nature we have hitherto* ad- 

 vised the use of the ocular only for 

 small pictures. By this is meant, 

 for example, such photographs as 

 will cover a plate not over four 

 inches square, or such as would be 

 required for making positives for the 

 projecting lantern by contact print- 

 ing. Thei'efore, a camera of the or- 

 dinary 4X5 size is quite large enough 

 for use with the ocular, and Prof. 

 Aubert, as will be seen further on, 

 speaks highl}- of the small Walker 

 camera, which takes plates only z'l 

 by 3?, inches, mounted al)ove the oc- 

 ular. 



As regards working without an 

 eye-piece, this is the plan followed 

 by Dr. Woodward in all his tine 

 work, for which the Army Medical 

 Museum became famous in his da}'. 

 The great advantage of this method 

 is seen when large plates are used, 

 requiring shai-p definition over a 

 .field ten or twelve inches in diameter. 

 A long camera is required for such 

 work. At the National Museum, 

 where 8 X 10 plates are used, the cam- 



shown in fig. I, or a similar cone 

 can be easily constructed of paper, in 

 the manner already described. The 

 length of the cone must be governed 

 by the size of the plate to be covered 

 and should be determined by experi- 

 ment. 



Although the ordinary view-cam- 

 eras may be used perfectly well, Mr. 

 Walmsley has devised a more elabo- 

 rate photo-micrographic and copving 

 camera, which, as its name implies, 

 is peculiarly adapted to this work. 

 It is represented in fig. 2. As will 

 be seen from the cut, the bellows is 

 capable of very considerable exten- 

 sion, the base being separable into 

 two parts. The ground glass has a 

 large circle of thin glass cemented in 

 the centre by balsam, which is said 

 to give a very fine screen for focussing 

 upon. The central part of the cam- 

 era opens by a door on the side, for 

 convenience in copying pictures, 

 making positives for the lantern, etc. 

 The plate-holder is single and opens 

 at the back (the most convenient 

 way), and it takes plates 4},X=^.V or 



3iX4i- 



In working in the manner here 

 proposed it is advisable to use an 

 amplifier, not merely to gain in mag- 

 nification, but for reasons that have 



Fig. I. — Scovill's Photo-Microscofic Equipment. 



era-bed is 5^^ feet in length. The 

 ordinary cameras are not made to 

 extend sufficiently, but a conical tube 

 of metal can be fitted to the front, as 



.* Vol. vi, p. 168. 



already been given but a short time 

 since.* We may, however, briefly 

 refer to the subject again in this place. 

 It will be obvious upon consideration 



* Vol.vi, p. 168. 



