34 



THE MICROSCOPE 



Fig. 23.— No. 3295, High-power Dark- 

 ground Illuminator. 



shows the optical portion of this illuminator. The light from 

 the mirror being thrown upon the under- surface of the glass 

 reflector, the light is reflected by two curved surfaces so that 

 a ring of light is focussed to a point upon the object at a very 



oblique angle, as shown 

 by the white portion of 

 the diagram. The whole 

 of this light is so oblique 

 that it will all be totally 

 reflected inside the glass 

 and will not emerge from 

 the illuminator unless 

 the latter is brought into 

 immersion contact with 

 the under-surface of the 

 slide by placing a drop 

 of cedar-wood oil be- 

 tween the top of 

 the illuminator and the 

 slide. It must be used 

 in immersion contact 

 with the slide in the 

 same way that an oil-immersion object glass is used in immersion 

 contact with the cover glass. With this iUuminator any dry 

 lens or an immersion lens with an aperture of less than 1 N.A. 

 can be used, and no direct light, but only that reflected by the 

 Use of oil- object, enters the microscope (see Fig. 24). A special oil- 

 objUcr^^"^ immersion 1/8-inch (3-mm.) focus, with an aperture of '95 N.A., 

 glasses with jg made for work with this illuminator ; or an immersion lens 

 groun . ^^^^ ^ larger aperture can be used if it be stopped down by 

 means of a small diaphragm placed behind the back lens of the 

 - object glass. 



In the latter case the object glass must be stopped down to 

 a considerably smaller aperture than 1 N.A., because the stop 

 cannot be placed in the best position, which is between the lenses 

 themselves, and with a stop behind the back lens a certain amount 

 of direct light is not properly excluded by a stop of the theoretical 

 size, because it is not in the correct position. 



There is a peculiarity in dark-ground illumination. The 

 object must be exactly at the crossing point of the beams of light — 

 that is, in its focus — or it will not be illuminated at aU (see Fig. 

 24), whereas with an ordinary condenser, even if the object is 

 not in the exact focus, it will still be illuminated, though, perhaps, 

 not so brifliantly. The non-focussing dark-ground illuminator 

 has no adjustment ; as the front portion of the illuminator must 

 be in immersion contact with the under-surface of the slide, it 

 cannot be moved up and down, and therefore the slides used 

 with this illuminator must be 1 mm. thick. SUdes of this thick- 



