196 OPTICAL PROJECTION 



In using immersion lenses, care must be taken to employ 

 a small drop of fluid, else it will drain away by its own weight. 

 Water lenses are convenient, but oil is far superior in bright- 

 ness and crispness of definition. A good Reichert oil lens 

 will work either direct, or with the low-power amplifier, as 

 well as with an eye-piece, up to 10 or 12 feet distance. 



97. Management of the Instrument. Before using the 

 Oxy-hydrogen Microscope, the alum-cell must be filled with 

 a saturated solution of alum. To make the solution add 

 common alum to hot water in excess, and allow the solution to 

 cool, then filter the clear portion twice through ordinary filter- 

 paper or blotting-paper. This solution may be used as often 

 as desired, but must be refiltered if any sediment or turbidity 

 appears, being used only in a perfectly limpid condition. 

 When replacing the alum-cell in the microscope, the side on 

 which the concave parallelising lens is balsamed should be 

 farthest away from the light. It is as well to leave the brass 

 stopper out of the cell when in use. As air-bubbles form on 

 the inside surfaces of the glass, they should be removed with 

 a camel-hair brush, as they impede the light, and sometimes 

 would appear as images upon the screen. They always appear 

 to some extent as the solution becomes heated. 



Not the slightest fear need be felt for the most delicate 

 slides on account of the heat ; but if preferred, the alum-cell 

 may be emptied and refilled with a fresh solution after an 

 hour's use, as it rapidly becomes hot. The cell should be 

 emptied after use, and washed out with clean water, or alum 

 slime may collect on the glass. 



Great care should be taken to procure a good light, the 

 full capacity of the best oxy-hydrogen jets being required for 

 work with high powers. 



The light from the lantern benig meant to be nearly parallel 

 after leaving the concave lens on the alum-cell, the first step 

 is to adjust the light to give this. This can be done after 

 gradually warming up the lantern and condensers, by taking 

 out the objective, mount, and sub-condenser, so as to leave 



