HIGH-POWER OR CRITICAL PHOTO-MICROGRAPHY 139 



at its summit by a very pronounced white tip, the so-called "pearl." Of course this 

 is an optical phenomenon, but it is very beautiful in some specimens. A small piece 

 is shown in the upper part of Fig. 5, Plate IV. 



In photographing the white dot plane of focus — the first we mentioned, Fig. 10, 

 Plate lY. — a narrow illuminating cone is required, and perhaps a touch of the 

 faintest kind of "oblique light," too much will make the black hexagonals look 

 elongated and distorted. With the black dot plane of focus (Fig. 5, Plate lY.) no 

 oblique light should be used, but the cone of light issuing through the condenser 

 should be carefully controlled by the iris so as to get the best effects without more 

 closure than possible. As we have before stated, it is not easy to get the definition 

 so perfect at this plane of focus, as will be seen by inspecting pictures taken by the 

 few who have given attention to the matter. Green light may be used with advantage. 



To refer again to the " pearl dots," we have personally never been able either to 

 see them well or to photograph them, unless the specimen was mounted in realgar — 

 hence we are led to suppose their presence is in some way due to the high refractive 

 index of the mountant. Use a narrow cone. Occasionally it may be necessar}/ to employ 

 an Edwards " Snapshot" plate, which is four times quicker than a " Medium " one. 



A somewhat peculiar point should here be mentioned before we leave this specimen. 

 It has been noticed by us that to obtain a photo direct of much above a 1000 diameters 

 is a most difficult thing, no matter what plane of focus, the result has never been what 

 we call good. We have seen the attempts of others, too, and have never been satisfied 

 with their results. The only way to get increased amplification is to re-enlarge 

 the negative, a plan adopted by several of the best photo-micrographers, including 

 Dr. Yan Heurck. It is curious to state this, because with our next specimen, the 

 Surirella gemma, there is no difficulty in doing so, even up to 3000 diameters. 



Surirella gemma (Fig. 9, Plate lY.). — Oblique hght is here needed, and a 

 specimen mounted in realgar best, and a somewhat narrow cone of light is wanted. 

 The little dots are mostly square, ovate, or circular, sometimes all shapes being found 

 in one specimen. They are about 50,000 to the inch. Owing to the amount of oblique 

 light necessary, it is better to use an Edwards "Snapshot" plate. White light, 

 exposure two minutes ; 1*35 condenser, well oiled to slip. 



Podura scales (Figs. 11 & 12, Plate lY.). — Scales of the Lepidocyrtis curvicollis. 

 Small cone, direct light, green glass, exposure about one minute. If studied they 

 will be found to exhibit curious markings — notes of exclamation " as they are called. 

 These simple black markings should be shown easily with a sixth, but of what we are 

 about to speak, and what we may, for convenience of description, call " the white interior 



