154 



Transactions of the Society. 



from that required for direct vision to that required for photo- 

 graphic projection, will obviously be to move the eye-piece out- 

 wards until the image formed by the objective lies sufficiently far 

 beyond the principal focus of the eye-piece. Some eye-pieces are 

 constructed in this way, being simply aplanatic magnifying glasses ; 

 in the commonest case, however, of the Huyghenian eye-piece, the 

 rays from the objective pass into the collecting lens of the eye- 

 piece before actually coming to a focus, and for these eye-pieces 

 the rational mode of focusing for photography would be to move 

 the back lens of the eye-piece only, provided that this lens were 

 individually corrected adequately as a photographic lens. A near 

 approach to this state of affairs is attained in the projection eye- 

 pieces supplied with a focusing scale, but even with these, the use 

 of the fine-adjustment is still required for focusing accurately. 

 In order to obviate the derangement of the objective when the 



Fig. 24. 



instrument is required for photography, the present Microscope is 

 fitted with a spiral focusing motion which enables the operator to 

 focus by moving the eye- piece alone— a process which makes it 

 possible to use ordinary eye-pieces for photographic purposes in a 

 much more rational manner. The movement of the eye-piece is 

 actuated by the rotation of a milled collar at the upper end of the 

 fixed body-tube, and this collar is grooved for the reception of a 

 driving band, whereby the operator can focus while observing the 

 ground glass screen of the camera. 



While the design of the instrument here described has been 

 arrived at entirely with the aim of providing an instrument 

 specially perfected for metallurgical purposes, it is recognised that 

 it may be desired to use the instrument for the examination of 

 sections or other objects by transmitted light. For this purpose a 

 special attachment is provided, consisting of a bracket which can 

 be attached to the stage bracket, carrying a right-angled reflecting 

 prism, iris diaphragm, and a swing-out condenser, with spiral 

 focusing motion, together 'constituting a high-class illuminating 



