68 PKACTICAL PHOTO-MICKOGKAPHY 



relation to the whole apparatus is not of primary importance. 

 The aperture J has to be centred exactly so that a parallel 

 beam is thrown on to the lens L, but after this has been done 

 the light may be thrown on to the mirror S 1 and is variable in 

 direction within considerable limits. 



An additional condensing-lens L 3 is also arranged, so that 

 it may be thrown into action over the aperture J if required ; 

 it enables the light to be condensed and a higher degree of 

 illumination to be obtained if required. The whole appliance 

 has been most carefully thought out, and forms at the present 

 time one of the most convenient outfits for opaque illumination 

 that can be obtained. At J an iris-diaphragm is placed so 

 that the width of the incident beam may be varied as desired. 

 The arrangement can, of course, be used at will with either the 

 prism or the cover-glass type of illuminator. 



Vertical illuminators require very considerable care when 

 used for photo-micrography, as there is great danger of a 

 considerable diffusion of light occurring at the object or at one 

 of the many reflecting surfaces in the optical system. For this 

 reason the best results are obtained by using these illuminators 

 in conjunction with oil-immersion objectives ; the number of 

 reflecting surfaces is thereby greatly reduced, and there is at 

 least no chance of light being diffused between the front of 

 the objective and the object, for the object is in this case 

 directly immersed in the cedar-wood oil. While these are 

 sometimes not of much importance for visual work they become 

 extremely troublesome when a photograph is required, and it 

 is absolutely essential that the exact conditions should be ful- 

 filled under which the appliance works at its best. 



A modification of the two forms described has been suggested 

 by Mr. Kosenhain. The general construction is similar to the 

 vertical illuminators described, but his reflectors consist of 

 differently shaped thin silvered surfaces, either circular or 

 annular, and they are mounted in such a manner that they may 

 be shifted either laterally or vertically. Great importance is 

 attached by Mr. Eosenhain to the necessity of having the 

 reflecting surface as close as possible to the posterior combina- 

 tion of the objective, and where particular appearances are 

 required the- adjustments suggested by which the reflectors 

 may be swung to one side or the other are extremely useful. 



