TELEPHOTO LENSES 139 



black ring in the negative, or white marks in the centre 

 of the print, as in Fig. C, Plate 12. These may be 

 effectually guarded against by covering any bright places 

 with a tube of dead-black paper or velvet. 



Advantage of the Wide-Tube Microscopes. The value of 

 the wide tube adopted almost universally on the better 

 microscopes becomes at once apparent when no eyepiece 

 is used, as the illuminated disc projected by an objective 

 is circumscribed by the tube, and it is frequently im- 

 possible to photograph a sufficiently large area of the 

 object with a narrow-tube instrument. The usual dia- 

 meter of the Continental eyepiece mount is 23 mm. and 

 the tube into wliich the eyepiece slips, or a diaphragm 

 within it, limits the pencil of rays from the objective, 

 and consequently the area of the object included in the 

 photomicrograph when no eyepiece is used. To increase 

 the field the draw-tubes are removed from the microscope, 

 and the diameter of the projected disc is then limited 

 by the outer tube, which varies from 30 mm. on small 

 stands to 42 mm. and even 50 mm. on the large ones. 

 Most objectives will not project a sharp image of their 

 entire field of view, and the area bounded by the widest 

 tube is often greater than they can satisfactorily cover. 

 But even so, the field is unnecessarily circumscribed by 

 the narrow tube ; and to make proper use of the small 

 photographic lenses, which are constructed with a view 

 to large covering power, and have a very large flat field, 

 even the wide tubes are none too large. 



Dissecting microscopes are made without tubes, and 

 can be conveniently used with a vertical camera when it 

 is desired to obtain the largest possible field and at the 

 same time make use of a microscope stand to carry the 

 object and lens. Some old-fashioned microscopes from 

 which the tube is removable are also suitable, and can be 

 used in a horizontal position as well. 



Telephoto Lenses. Telephoto lenses can be used to 



