MEDIUM-POWER PHOTO-MICKOGRAPHY 113 



6. The Illumination of Opaque Objects. — It is obvious transmitted light 

 will be of no service as the object is mounted opaquely. To illuminate this type of 

 slide it is often necessary to remove the cover-glass, for the reflections caused by it, 

 although not so noticeable visually, are very detrimental photographically. With 

 the limelight it is necessary to use a condenser, the auxiliary does very well, and 

 with them a bath to prevent the sHde being melted with the heat. Many objects 

 require two limelights, one on each side, each being provided with condenser and 

 water-bath. This form of illumination is sometimes called "double oblique light." 

 It has nothing to do with "oblique light" previously mentioned on page no, which 

 has already been pointed out. The great difiiculty in illuminating opaque objects is 

 to prevent the objective casting a shadow on them. With high-powers as they have 

 to be so close, this is almost impossible to avoid, and for that purpose Zeiss has brought 

 out a special form of illumination, Fig. 63, page 113, consisting of a prism placed above 

 the objective which is illuminated from the side. The light passes through the 

 prism and the objective and thence on to the object and back again through the 

 objective to the eye lens. This arrangement has not answered our expectations any 

 more than a large reflector we had made — Lieberkuhn fashion — to throw the light 

 by reflection on to the object. An illustration of the photography of an opaque object 

 is shown in Fig. 6, Plate V., and of " double oblique illumination " in Fig. 4, Plate II. 

 It was a particularly difficult object as the hairs stood up quite a sensible distance 

 and to obtain depth of focus suflicient for the general eflect was no easy matter. 



7. Illumination by a Large Cone, Small Cone and Hollow Cone have 

 been explained on p. 92. 



8. Monochromatic Light, Partial and Complete. Partial monochromatic 

 light is obtamed by the use of coloured glasses interposed between the illuminant 

 and the substage condenser, or between the condenser and the object. In the blocks 

 of our apparatus. Figs. 24A, B and c, page 54, the Btinsen holder is seen holding a 

 strip of glass. 



Monochromatic Light completely so called cannot be obtained by the use 

 of any glass we know of. It would be a great convenience if it could. This can only 

 be procured by the use of a prism of dense glass to break up white light into its 

 component colours. This method gives colours of great purity and definite wave 

 length, but is difficult to arrange, because the beam is too narrow to anything like 

 fill the field of the microscope, and cannot in some cases be made to do so even with 

 the auxiliary condenser. For that reason a more powerful set of prisms has to be 

 employed, giving greater dispersion, bat the illumination then is so feeble that, unless 



