112 



PHOTO-MICROGRAPHY 



in any azimuth he may desire. It is an exceedingly ingenious arrangement and one 

 that answers admirably. 



When the use of the oblique Ught is finished with, the centrality of the iris 

 diaphragm with the axis of the microscope, is known to be re-obtained by turning the 

 same handle backwards until a distinct click is heard, such click gently restraining 

 all further movement and audibly indicating the return of the parts to their original 

 centrality. 



Oblique light must not be confused with what has been called " double oblique 

 light " for they are quite different, as pointed out later on page 1 1 3. 



4. Annular Light. — This has been already referred to on p. 94, and is obtained 

 by stopping out the centre of the condenser ; and is the same practically only to a 

 different degree as dark-ground illumination when produced by a " spot lens." ^ 



5. Dark Ground Illumination is said to be obtained when the object which is 

 mounted transparently, is supplied with a dark ground optically produced, although 

 itself remaining brilliantly illuminated. The effect produced by this means with low- 

 powers is at times exceedingly fine. It may be obtained by (i) the "spot lens" as 

 just described, and on p. 102, the Davis diaphragm being closed a little to complete 

 the effect ; (2) by use of the stop placed beneath the condenser which may be made of 

 different sizes to suit the lens used, or the special form designed by ourselves, p. 95, 

 Fig. 48, and (3), by the use of an arrangement called the Paraboloid. This consists 

 practically of another form of condenser which is fitted into the substage in the 

 place of the ordinary condenser, having a black central disc of brass capable of being 

 pushed up and down independently of any movement of the condenser itself Con- 

 siderable experience is necessary to thoroughly master this ingenious arrangement 

 designed by Mr. Wenham. Having obtained a good illumination of the object the black 

 metal disc is pushed up or pulled down and the Davis diaphragm closed until the 

 black background is obtained. In photographing this effect it is not well to obtain 

 the field as black as when the arrangement is used for visual purposes ; especially 

 when a lantern slide is desired to be shown on the screen. The effect is spoilt if 

 the back-ground is too completely black. This is not so, however, in a print, or 

 when visually using the microscope. It may here be mentioned lantern-plates do 

 not give usually such good results as an Edward isochromatic negative plate. 

 About one second exposure required or perhaps less at i foot from a gas-burner. 

 An example is given Fig. 5, Plate V. 



* Condensers are sold which admit of a piece of black paper being affixed to the lens nearest the hght— hence the 

 n^me "spot lens," 



