246 HOW TO WORK 



Dr. Maddox finds the general appearance of the image and the| 

 condition of the field to be best seen on the card ; therefore he uses) 

 this placed before the screen and resting against the frame, to procure) 

 the primary focussing, the final adjustment being made by the rod 

 and fine motion, when examining the image by the focussing glass, 

 on the slightly opaque screen, or on a thick card substituted for it, 

 using a hand magnifier to examine the image, which he prefers. 



In front of the exposed plate is arranged a diaphragm to exclude 

 extraneous light, while the parts about the stage of the microscope 

 are well protected from the light diffused through the slide from the 

 sub-stage condenser. 



Although this plan is very convenient, and allows everything to 

 be ready at hand, it may be as well to point out some of its disadvan- 

 tages. It is difficult to see the state of the sky ; hence, after placing 

 the sensitised plate in the frame, a cloud approaching unnoticed 

 may at once obscure the sun and cause the loss of the plate. In 

 the wet collodion process, dust is liable to settle on the surface. 

 Again, for opaque objects which require a side illumination, as 

 in the combined images for the stereoscope, the necessary devia- 

 tion must be procured by prisms. Even if the Lieberkuhn with a 

 portion of its surface stopped out, and the dark wells or stops, 

 spot lens, or M. Nachet's cone be used, there will be considerable 

 danger from leakage of light and a fogged plate. Moreover, the 

 eyes become fatigued if kept long under yellow light. Some persons 

 even complain of giddiness. 



The plan for using some form of draw camera is to a great extent 

 free from these defects, and the method proposed by Dr. Moitessier 

 in his work, to which allusion was made in the early part of this 

 chapter, appears so useful, that I shall briefly notice the chief points. 

 The microscope arranged horizontally, with a grooved bar projecting 

 beyond the base-board to carry the mirror, sulphate of copper cell, 

 ground glass, and diaphragm, is centrally attached to an expanding 

 camera. The dark box or part where the focussing screen is placed, 

 has one of the sides to open with hinges as a door, and the operator 

 seated by the side of the instrument, with or without a cloth drawn 

 over the head to exclude the surrounding light, examines the image 

 from the side opening, either with or without a magnifying glass, the 

 right hand being occupied in the necessary arrangement of the 

 object and the illumination ; the plate being ready in the dark slide, 

 and the side door closed light-tight, it is inserted and exposed without 

 loss of time. 



Dr. Moitessier has likewise recommended a slide with adjusting 

 motions, so as to expose different parts of the sensitised plate one 



