256 PEACTICAL PHOTO-MICKOGKAPHY 



essentially one that should not be attempted by a novice, but 

 only by those who have already had a fairly wide experience 

 in other branches of the work. It presents all the usual 

 difficulties of dealing with transparent preparations, with the 

 additional one that in no case can the direct image be dealt 

 with ; in fact, it is only when the photograph is actually 

 developed that a definite idea as to whether success has 

 been attained or not can be arrived at. 



The Photography of Metal Surfaces. To obtain photo- 

 graphs of such objects certain special conditions have to be 

 complied with. The main point of difference between these 

 and transparent objects is in the method of illumination. In 

 the section dealing with the various appliances for efficiently 

 illuminating objects, some description has already been given 

 of what are known as opaque illuminators, and it is in the 

 correct use of these appliances, and so obtaining proper 

 illumination of the object, that the chief difficulty exists. 

 It is also necessary to have in the microscope itself certain 

 modifications of construction to enable the object and the 

 additional optical appliances to be dealt with more easily. 



A further difficulty lies in the method of focussing. It is 

 obviously a great disadvantage, where an ordinary stand is 

 employed, if the position of the object cannot be altered 

 without disturbance to some of the illuminating arrangements. 

 Fig. 70 shows the general arrangement when a metal surface 

 is illuminated by means of the vertical illuminator. From 

 this it will be seen that the illuminant A is a fixed point, 

 and that the position of the reflecting-mirror B at the back 

 of the objective should be constant. To enable this to be 

 done it is therefore advisable for the stage of the microscope 

 itself to have a coarse adjustment, and that an accurately 

 made one, so that the object may be brought up to its position 

 in correct focus, leaving only the final exact focussing to the 

 fine-adjustment, if such is necessary at all. 



A typical instrument of this class is the Zeiss large photo- 

 micrographic stand arranged for either transmitted or reflected 

 light. This instrument is the usual standard type for photo- 

 micrographic work already figured and described ; but it has 

 in addition a coarse-adjustment, which moves the whole of the 

 stage and sub-stage apparatus. It also has attached to one 



