186 



PKACTICAL PHOTO-MICEOGKAPHY 



If reference is made to Fig. 47 one method of arranging 

 such an opaque screen is seen. It consists essentially of a piece 

 of plain board which is hinged on to the side of the frame 

 carrying the focussing-screen of the camera, and on the side 

 towards the microscope is fastened a piece of plain white paper 

 or cardboard. The white surface is in alignment with the focus- 

 sing-screen of the camera, but as it is hinged the position may 

 be altered when the microscope is swung out. Obviously this 

 screen must be at a different inclination to the optical axis 

 according to the length of camera in use, so that the hinges allow 

 of variation of the inclination. The image may be approxi- 

 mately projected on to the opaque screen in the position in 

 relation to the microscope that it would oc- 

 cupy on the ground-glass focussing-screen. 

 With this arrangement it is possible to 

 make all the adjustments centreing of the 

 sub-stage of the microscope, centreing of 

 the object, focussing of the projection- 

 ocular diaphragm, approximate focussing 

 of the object itself and to observe finally 

 if all the adjustments are correct, while 

 standing in a position near the micro- 

 scope. It is obvious that this renders the 

 work of adjustment very much easier, and 

 any correction can be made without having 

 to move backwards and forwards from 

 the ground-glass screen to the microscope, while the effect of 

 any small alteration that may be desirable is immediately 

 seen. On this, as well as on the ground-glass screen in 

 the camera, diagonal lines should be drawn so that their 

 point of intersection indicates the position that should be 

 occupied by the centre of the object. It is also useful to make 

 a circular mark that indicates the largest image that may be 

 photographed on the sensitive plate to be used. In the case of 

 a quarter-plate this circle could not be more than three inches 

 in diameter. 



Focussing Eye-pieces. When using the plain-glass focus- 

 sing-screen an achromatic eye-piece, usually of the Kamsden 

 type, must be employed. There are many types of these on the 

 market, and the one illustrated (Fig. 63) is typical. It must 



FIG. 63. Focussing- 

 (Zeiss.) 



