PHOTOMICROSCOPE : J. H. G. MONYPENNY 143 



for the purposes of microscopic work may be looked upon as an 

 illuminant. There is one essential point, however, the beam of light 

 thrown by the lens must cover the whole surface of the back lens 

 of the objective, otherwise some of the aperture of the latter is lost. 



In Fig. 3, A represents a lens of about 6 in. focus, placed at 

 the requisite distance from the microscope to give critical illumina- 

 tion, as described earlier; the objective therefore uses this lens as 

 the illuminant, and forms an image of it in the field (if any slight 

 scratches are present on the surface of the lens, it should be moved 

 very slightly out of focus). Condenser B (about 25^ in. focus and 

 2j/8 in- diameter) is placed at such a distance from A that the 



■ Troujtl 



Fig. 3- 

 Condenser System No. 2. 



latter focusses B approximately on the objective. Finally B forms 

 an enlarged image of the illuminant on A. This sounds rather 

 complex, but the result is that if the eye be placed at A, B appears 

 as an even disc of light, and similarly to an eye placed against 

 the objective A appears as an even disc. As mentioned above, the 

 only point that must be carefully watched is that the image of B- 

 formed by A on the objective must at least cover the back lens of 

 the latter. The figures given above refer to the author's arrange- 

 ment, and in this case the image is rather more than two-thirds of 

 an inch diameter, and therefore amply large enough for any objective- 

 in use. Condenser A need not be more than 1 in. diameter, but 

 the image of the light formed on this lens by B should completely 

 fill it. This condenser (A) should have an iris diaphragm for 

 limiting the area of the field illuminated as described earlier. An 

 iris is not required on condenser B, except for centering and focussing 

 purposes. 



Using this arrangement, one is able to illuminate evenly the- 

 section, and also provide critical illumination. It will be found 

 that if condenser A is used without B, unless the illuminant has- 

 a large area or is placed very near to A, the conditions of critical 

 illumination are not obtained, the beam of light not being sufiicieni 

 to fill the back lens of the objective. The effect is equivalent to- 

 cutting down the aperture of the objective, with all the bad effects 

 produced thereby. The author has seen more than one metallurgical 

 photomicroscope in use in which this condition of things has obtained. 



It is obvious from Fig. 3 that, using an illuminant of small area, 

 such as the Pointolite, condenser B should have a short focus, and 

 also the better corrected it is the more light will be available — the 

 author uses the Watson-Conrady condenser, and finds it excellent 



