142 SOME NOTES ON THE METALLURGICAL 



As regards the illuminant, the author prefers the 500 C.P. 

 Pointolite Lamp (a tungsten arc lamp made by the Ediswan Co.) to 

 any other type of lamp made; the intensity of the light is very 

 great, and it is absolutely steady. It requires direct current, and 

 where this is available the author has no hesitation whatever in 

 recommending it in preference to any other form of illuminant. 

 Previous to this lamp being on the market (about 1917), the author 

 had tried a Nernst lamp, an arc lamp, and lime-light, and had for 

 some years used the last in preference to the first two. The intensity 

 of the light given by the lime is not nearly so great as with the arc 

 lamp, but on the other hand it is perfectly steady, and this cannot 

 be said of the arc lamp. 



Coming to the condensing system, probably one of the simplest 

 arrangements is that shown in Fig. 2. In this case the condenser 

 C forms an enlarged image of the illuminant L at Li, the correct 

 distance from the illuminator to give critical illumination as described 

 earlier. By this means the area of the illuminant is spread out, 

 and with, for example, lime-light, one may obtain perfectly even 

 illumination even when photographing at, say, 5 or 6 times the 

 initial power of the objective. The effect of any slight irregularities 



^ 



Fig. 2. 

 Condenser System No, i. 



on the surface of the illuminant may be avoided by forming the 

 image Li about 1 in. nearer the microscope than its correct position, 

 as mentioned earlier. It is advisable to have an iris diaphragm 

 at Li and to close it until only slightly more than the area to be 

 photographed is illuminated. This cuts off a lot of stray light which 

 would otherwise reduce the contrast by giving a general fog over 

 the whole section. This iris should be focussed fairly accurately 

 on the section, otherwise there is a gradual falling off of the illumina- 

 tion on the edge of the field instead of a sharply defined edge to 

 the illuminated area. 



While this method is perfectly satisfactory for use with lime-light 

 — the author has taken several hundred photographs at magnifica- 

 tions ranging from 30 to 2,000, using an arrangement of this 

 description — it has certain drawbacks; for example, a great deal 

 of light is wasted, and with an illuminant of small area it is difficult 

 to fill the field evenly unless a very long optical bench is used. 



These defects are obviated in the following arrangement. In 

 this, advantage is taken of the fact that if a biconvex lens is held 

 between the eye and a light {e.g., a candle flame) in such a manner 

 that the eye and flame occupy the position of conjugate foci then the 

 lens itself will appear to the eye as an evenly illuminated disc, and 



