PLATE 43. 



Resolution of Diatoms by means of Oblique 

 Illumination. 



A B 



Surirella gemma A mphipleura pellucida 

 x 1620 x 1250 



Objective Zciss 2 mm. Apo. N. A. 1.30. 



Ocular Zciss Compens. 8. Zciss Projection 4. 



Condenser ... Watson's Universal Holos. immersion 



N.A. 1.0. N.A.1.34. 



Screen None. Wratten'sH (blue). 



Illuminant ... Liliput 4 amp. arc. 



Aux. Condenser Double convex lens, giving parallel beam. 



Plate Imperial Special Rapid Ort ho. 



Developer ... Pyro-soda. 



Exposure 1 min. 45 sec. 



A crescent or slotted stop was placed in the ring beneath the 

 substage condenser, to direct the light obliquely on to the diatom, 

 in a direction determined according to the principle stated on 

 page 178. Parallel to the length of the frustiile in the case of 

 Amphipleura to show the transverse striations. 



C Amphipleura pellucida x 3750 



From a bromide enlargement of the same negative as B. 



It is a very delicate matter to get a sharp photomicrograph at such 

 a high magnification as this, and as a rule the simplest process is to 

 take a negative at a lower amplification, provided that all the detail is 

 resolved, and to enlarge it, as in this example. 



The photomicrograph of S. gemma shows the white dot focus; 

 a good objective reveals the dots very sharply defined. A. pellucida 

 may also in certain cases be resolved into dots, or, by altering the 

 direction of the light, into longitudinal striations, but the resolution 

 depends so much on the individual specimen that its value as a test 

 object, is less than it would otherwise be. 



Photos. W. B. R. i Jo follow Plate 42 and page 256. 



