WITH THE MICROSCOPE. 243 



These apertures are numbered according to the inches i, 2, 3, &c., 

 from the frame, and thus are of service to note the distance at which 

 the sensitised plate is placed from it or from the stage. Memoranda 

 being kept, the same ranges can be easily repeated. The draw 

 chamber of the camera has its own focussing screw which is of 

 use occasionally, but it is not necessary. 



Two diaphragms of blackened stout card are placed within the 

 chamber of this elongated camera, one near to the vertical frame or 

 at the junction of the bellows part with it in front, and the other is 

 placed in a grooved frame, that slides in a wide cut made in the 

 inner surface of the underside of the draw part of the camera. This 

 frame holder takes diaphragms with various sized apertures, according 

 to the dimensions of the image of the object or the glass plates 

 employed. Sliding this forward or backward in the camera alters the 

 relative size of the field according as the camera is used expanded 

 or closed. The camera is either dead-blackened, or lined with black 

 cotton velvet, and the tube of the microscope inside is well covered 

 with optician's charcoal black, or lined with black velvet, which is 

 better. 



The mirror or prism is set on a separate arm fixed to the base 

 board in a line with the stem of the microscope, so that the axis 

 shall correspond with the axis of the objective. The apparatus can 

 be put together very quickly, or kept ready for use, o nd is of a size 

 that permits of its being moved about easily, without being too cum- 

 bersome for one person ; and it possesses considerable firmness. 



The microscope portion can be supplied by any form of micro- 

 scope that will take the horizontal position, and permit the eye-piece 

 end of the body to work through the central aperture in the front of 

 the bellows-chamber, provided means are taken to effect rigidity, and 

 completely shut out the outside light around the aperture when 

 working the rack for the coarse adjustment. But preference has 

 been given by Dr. Maddox to a tube shorter than the usual body 

 of the ordinary microscope, which sometimes narrows the field too 

 much when the camera is nearly closed on the vertical frame. The 

 tube consists of two parts, one an inch in diameter fixed to the arm, 

 the other i| inches in diameter, that slides through the aperture in 

 the door. On the open end of the latter fits a dead blackened brass 

 cap, from the inside, with a slight internal projecting ledge, which 

 acts as a diaphragm with a large opening. 



The description will be more easily understood by a reference to 

 pi. LIII, which represents the instrument partly in section. The 

 camera, when drawn out to its full range, exhibits this objection : the 

 operator is obliged to withdraw the head from the focussing screen 



R 2 



