SECTIONS AT HIGH MAGNIFICATION 159 



Photomicrographs of Diatoms at 5,000 magnifications and over, 

 taken by transmitted light, have been published ; but so far as we are 

 aware steel sections at such a high magnification have not 

 been available. This may be easily accounted for by the difficulties 

 in the way. Although, unfortunately, we are unable to indicate 

 an easy path by w^hich these difficulties may be avoided, we propose 

 to show the means by which we endeavour to overcome them. 



We have already laid stress on the need for an objective of high 

 resolving power capable of giving good definition when combined 

 with a properly compensated eyepiece of high magnifying power. 



Probably the next most essential point is that the specimen to be 

 photographed be properly etched. Deep etching is fatal ; it causes 

 pits and furrows in the surface of the piece which extend beyond the 

 range of depth of focus, which with a high power objective, is naturally 

 very limited. Therefore the most delicate etching is necessary, and 

 this we find is usually best obtained with 5 per cent. Picric Acid in 

 Alcohol. " _ 



The illumination of the specimen for photography is the next 

 subject for attention. For high power photography the lighting 

 should be as intense as possible. We use a 20 ampere arc lamp of the 

 hand fed type, and this is found preferable to one mechanically 

 fed. It is simple, has no mechanism to get out of order, and the 

 carbons are not liable to re-adjustment at the critical moment, just 

 when the plate is being exposed. Moreover, mechanically fed carbons 

 are never so firm and free from vibration as those of the hand fed 

 types. Alternating current at about 70 volts can be used with perfect 

 success on a 20 ampere hand fed lamp, if cored carbons are used. 

 This is a point on which emphasis should be laid, for the makers of our 

 apparatus have always laid stress on the necessity for direct current for 

 photomicrographic work. Tungsten Arc and Mercury Vapour Lamps 

 have been more recently introduced for photomicrographic work, 

 but we have had no opportunity of testing them. 



The vertical illuminator attached to the Microscope should be a 

 plain glass disc. We find a prism unsatisfactory for this work. The 

 light should be focussed on the diaphragm of the vertical illuminator, 

 and of course it must be perfectly central with the Microscope and 

 the camera. 



The iris diaphragm of the illuminator should only be closed as much 

 as is necessary to get sufficient of the field sharp. Further closing 

 of the diaphragm not only interferes with the resolution, but produces 

 false images. An example of this effect is shown in Figs. 

 10 and II, Plate D. Fig. 10 illustrates the result produced by 

 closing the diaphragm too much, and Fig. 11 shows a correct image 

 obtained by proper adjustment. 



The diaphragm in the condenser system should be closed so that 

 only the area to be photographed is illuminated. 



For apochromatic objectives, a blue screen as a light filter should 

 be used, and ordinary photographic plates. The specimen is focussed 

 first of all on the ground glass screen of the camera, and finally adjusted 



