ULTRAMICROSCOPY 



63 



of apparatus satisfying these conditions was invented by 

 Siedentopf and Szigmondy in 1903. ' The arrangement is 

 shown diagrammatically in fig. 2. 



The light from the source — sunlight focussed by means of 

 a heliostat, or an arc-lamp — is focussed by a lens A on to a 

 slit B. By means of C and D a reduced image is formed on 

 the axis of the observing microscope E and in the preparation 

 to be examined. The slit B is adjustable in both directions, 

 and in some the dimensions can be reduced so that the length 

 is about o"2 mm., the width only 0*05 mm. The lens C and 

 the feeble microscope D gives an image which is only about 

 ■g\ of the dimensions of the illuminating bundle ; therefore the 

 illuminating beam at the centre of the field of the microscope 

 has a minimum width of about 0*003 mm - or 3 A* ana " a minimum 



Fig. 2,. 



depth of 1 '5 p. The intensity of illumination is therefore very 

 great and the only light entering the microscope is diffracted 

 light. The background is therefore dark. For greater efficiency, 

 it is obviously better to arrange that the depth of the illuminated 

 field is equal to that of the microscopic field, which, of course, 

 for microscopes of high magnifying power is very small. 



It is often necessary to know the volume observed : this can 

 readily be determined in the following manner. The precision 

 slit is placed horizontally and the width of the microscopic 

 field is read off on the scale in the eyepiece. The slit is 

 then rotated through 90° and a second reading taken ; this 

 gives the depth of the layer illuminated. The illuminating 

 bundle is, of course, spread out a little in the field of the 



1 This has usually been recognised ; but in the Phil. Mag., April 1909, there 

 appears a letter from Prof. Ranan, of Calcutta, referring to an apparatus of 

 Mr. G. Dubern which he claims is almost identical with Cotton and Mouton's 

 modification of Siedentopf s apparatus. Dubern's papers appeared in the weekly 

 journal Indian Engineering, April 7, 14, and 21, 1888. 



