ARRANGEMENT OF APPARATUS. 115 



round disc of light formed on the ground glass is 

 placed exactly central by means of the levelling 

 screws at each corner of the oblong board, to which 

 the camera is attached. When this is accomplished, 

 the binding screw is tightened, and the conical bel- 

 lows slipped into its original position on the front of 

 the camera. Great care is now required in attaching 

 the end of the conical bellows to the short tube of 

 the microscope. It will be much more easily done if 

 the interior of the tube fixed to the end of the coni- 

 cal bellows is covered with black velvet, as well as 

 its interior ; it will then slip easily into that of the 

 microscope, and the entrance of all light rays will be 

 prevented. 



The image of the object is roughly focussed on 

 the ground glass ; the desired magnification ob- 

 tained by racking the" ground glass backwards or 

 forwards to increase or diminish it, bearing in mind 

 that the actinic power of the light is said to decrease 

 in proportion to the square of the distance from its 

 source. 



The part of the object we wish to focus should, if 

 possible, completely fill the plate ; this is sometimes 

 impossible, as its thickness prevents the use of len- 

 ses of sufficient power to magnify the lateral dimen- 

 sions, while their penetration defines those parts ly- 

 ing on different planes. 



The most satisfactory magnification being ob- 

 tained, we pass the silk cord round a bevelled ring 

 attached to the fine adjustment of the microscope, 

 and the wheel on the rod passing under the camera 



i2 



