^joi.rna'i.SrS'm'] «s a]ipliecl to Photo-micvograpluj. 295 



the microscope (whicli has previously been levelled), is raised or 

 lowered and moved from side to side till the centre of the achro- 

 matic condenser is brought to the centre of the illuminating pencil 

 proceeding from the lamp ; the object is then placed on the stage 

 and carefully adjusted. A cell of plate glass containing a saturated 

 solution of the ammonio-sulphate of copper is fixed just below the 

 achromatic condenser, and not only prevents the admission of non- 

 actinic rays, but excludes the very great heat which accompanies 

 the Electric light, and also moderates its efiect upon the eye of the 

 observer. The light thus produced is very agreeable to the eye, 

 and I find myself able to work with it from four to five hours with- 

 out fatigue. It has also the advantage that all the colours of the 

 object examined disappear, and the preparation appears black on an 

 azure field which resembles the sky on a clear day, so that the 

 observer sees at a glance how the object will appear in the photo- 

 graph (in which the same black lines or tints will be faitlifully re- 

 produced on a white field), and is thus enabled to arrange his achro- 

 matic condenser and other adjustments so as to produce the most 

 satisfactory efi'ect. 



Everything having been arranged at the microscope to the satis- 

 faction of the observer, the eye-piece is taken out, and, the image 

 allowed to fall on the ground glass of the plate holder, which has 

 previously been placed at the distance necessary to give the magni- 

 fying power desired with the objective employed. The operator 

 adjusts the plate holder to the right height and sees that it is per- 

 pendicular to the optical axis of the microscope, which he readily 

 does by observing that all parts of the field are equally in focus. 

 He then takes out the ground glass and finishes the fine adjustment 

 with a sheet of plate glass and a focussing glass, after which the sensi- 

 tive plate is inserted, the exposure made, and the operation is finished. 



To enable the observer to focus the microscope while sitting at 

 a distance from it at the sensitive plate, the following contrivance 

 is employed. On the table which supports the microscope (e) two 

 brass shoulders, each 2 inches high, are screwed. Through these 

 runs an iron rod 9 inches long, on which slips a brass pulley {d) 

 which can be clamped at any point. A cord connects this pulley 

 with the wheel of the fine adjustment of the microscope, which is 

 grooved for the purpose. It is evident that whenever this iron rod 

 is turned, the pulley turning with it will move the fine adjustment 

 of the microscope. To effect this, the iron rod terminates in a 

 square extremity, so that a joint of an ordinary fishing-rod, to 

 which a brass ferrule, shaped like a watch-key, has been riveted, 

 enables the operator to focus the microscope at any ordinary dis- 

 tance. When greater distances are required, two joints of the rod 

 may be used. The rod, being graduated into feet and inches, en- 

 ables the operator to record the distance employed for each picture. 



