300 PEACTICAL PHOTO-MICEOGEAPHY 



PLATE VIII, FIG. 1. Diatom : Navicula lyra. x 850. 

 Objective, 4-mm. apochromat, by E. Leitz. (Tube-length, 

 170mm.) 



Ocular-projection, No. 4. 



Uluminant, lime-light, mixed jet. 



Condenser, Swift apochromatic, N.A. 0*95. 



No auxiliary condenser. 



Screen, Gifford F-line. 



Plate, Wellington process orthochromatic. 



Exposare, 35 seconds. 



Development, pyro-soda. 



PLATE VIII, FIG. 2. The Podura Scale (Lepidocyrtus 

 curvicollis). x 1750. 



Objective, - x g-inch achromatic. (Tube-length, 168 mm.) 

 By J. Swift & Son. 



Ocular, Swift mediar No. 3. 



Uluminant, electric arc, 4 amperes. 



Sub-stage condenser, Swift apochromatic stopped down 

 to one- third cone. 



Auxiliary condenser, Zeiss No. I. 



A parallel beam was projected into the sub-stage condenser 

 by means of the auxiliary condenser, and a critical image of 

 the iris-diaphragm in front of the auxiliary condenser projected 

 into the object-plane. 



Screen, Zettnow's fluid filter, 6 mm. thick. 



Plate, Wellington orthochromatic process. 



Exposure, 45 seconds. 



Development, pyro-soda. 



This object has already been mentioned as being the 

 recognised optician's test for adjusting objectives. To obtain 

 a perfectly defined photograph of it, the objective must be of 

 the finest preferably, apochromatic ; but if a suitable green - 

 glass screen is used, such as Zettnow's filter already described, 

 or a signal-green and pale-yellow glass combined, a good 

 achromatic oil-immersion objective will suffice. 



The point to bear in mind in attempting this object is that 

 the internal exclamation marks are as clearly shown as possible ; 

 they should have a knoblike end, and then taper off from that 

 to a fine point. A one-eighth-inch oil-immersion objective 

 would show these equally well. The object is a test for 



