ARRANGEMENT OF APPARATUS, ETC. 



103 



coincident with its optic axis. Perfect accuracy of 

 adjustment in this respect, is one of the greatest 

 aids to the production of good and truthful nega- 

 tives* 



The object, chosen by the rules laid down in the 

 following pages, is placed on the stage and brought 

 into focus. If the light is too strong, the distance 

 of the diaphragm is increased, or a smaller aperture 

 substituted, taking care that it occupies exactly the 

 same position with regard to the optic axis of the 

 apparatus as the former one, and vice versa. It 

 should be born in mind that the less light incident 

 on the object, consistent with clear definition and 

 subsequent facility in focussing, the better. 



It would be much simpler to centre the tube 

 holding different sized discs, as they might be sub- 

 stituted for each other without any danger of alter- 

 ing the position of the other parts, but withdrawing 

 the inner tube would very likely displace the mirror; 

 this plan is more suitable when the apparatus is 

 placed in the horizontal position, no mirror being 

 then necessary. Whichever plan is adopted, the 

 differentiation of the structure should be distinct; 

 the markings dark; their interspaces bright and 

 clear. The attainment of the proper means will be 

 facilitated by remembering that too great a quantity 

 of light impairs definition ; too little increases the 

 difficulty of focussing and unnecessarily prolongs the 

 exposure. Intensity is what is required, not quan- 

 tity. 



The best light being obtained, the eyepiece tube of 



