2l6 PRACTICAL MICROSCOPY. 



from the stage of the instrument. Dr. Woodward gives 

 the preference to immersion objectives, the corrections of 

 which are generally well suited to photographic require- 

 ments. 



With a \ objective and the arrangement above de- 

 scribed, the field is so brilliantly illuminated that the eye 

 cannot safely be permitted to look down the tube. The 

 image is therefore received on a piece of white cardboard, 

 and, sitting by the microscope to make the adjustment, the 

 card is viewed with both eyes, precisely as in the case 

 of the ordinary solar microscope. 



When all is satisfactory, an ammonio-sulphate cell is 

 inserted between the large lens and the achromatic con- 

 denser, and the velvet hood, which prevents leakage of light 

 from about the microscope into the dark room, is drawn 

 down.. 



Dr. Woodward finds it best to use the naked objective 

 without eye-piece or amplifier, and not as a rule to fix 

 the sensitive plate more than 3 or 4 feet from the stage of 

 the microscope. A \ objective may be conveniently 

 employed to obtain powers of from 200 to 500 or 600 

 diameters; a T V for higher powers, up to 1200 or 1500 

 diameters. 



It occurred to him that for such short exposure the 

 heliostat might be dispensed with, and on trial without it 

 found that a large right-angle prism used in the position of 

 total reflection, or even an ordinary mirror, gave excellent 

 results ; the exposures being even shorter than when the 

 heliostat was used, since there was but a single reflection. 

 Under ordinary circumstances the heliostat appears desir- 

 able for general use since, the solar pencil being thrown in 

 a constant direction, the trouble of adjusting the illumina- 

 tion of a series of objects is considerably diminished ; but 

 equally good pictures can be produced without it, even 



