ARRANGEMENT OF APPARATUS, ETC. 117 



by perseverance, we prefer repetition on our own 

 part, to failure on that of our readers. 



The following arrangement should be selected by 

 those compelled to adopt one plan alone. Its advan- 

 tages are, the ease and quickness with which the 

 different manipulations are performed; the small 

 number of adjustments necessary ; and the absence 

 of all danger of disturbing the centering of the 

 different instruments, as the microscope and camera 

 are not placed in direct continuity. In the former 

 arrangement this was very liable to occur, when fix- 

 ing the bellows front to the camera and microscope, 

 or when substituting the dark slide for the focussing 

 screen. 



Perhaps its greatest advantages are the possibility 

 of focussing at any distance from the microscope and 

 of manipulating without leaving the dark room. 



The drawback connected with this method, is the 

 impossibility of photographing objects in fluid, on 

 account of the horizontal position of the microscope. 

 For this reason also we should be extremely careful 

 to accurately arrange the distance of the condenser 

 from the slide, because if the heat rays are brought 

 to a focus on the object, even when an ordinary parafin 

 lamp is the source of illumination, those mounted in 

 balsam are liable to change their position, owing to 

 the melting of the medium during exposure. 



The recess jutting into the dark room, full descrip- 

 tion of which has been already given, being con- 

 structed, the entrance of all actinic rays is pre- 



