ARRANGEMENT OF APPARATUS. 129 



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 

 cf 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, (the heat being greatly 

 increased owing to the absence of a reflecting sur- 

 face even when an ordinary parafin lamp is the 

 source of illumination), objects 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- 

 vented, by pasting several folds of yellow or black 

 paper, (the latter being preferable) over both interior 

 and exterior. The edges of the sleeves where they 

 join the partition are further secured by tacking thin 

 deal laths round them, which are also covered with 

 black paper. A black cloth curtain with an orifice 

 for the tube of the microscope and fine adjustment, 

 should be placed inside the dark room so that it can 

 be let down when the arrangement of the apparatus 

 is complete. 



We suppose the apparatus continuing in the posi- 



