April — iipt. 1859.] of Microscopic ohjtcts. 



15 



ed upon the focusing screen ; with the deeper eye-pieces, the field 

 is from 8 to 9 inches, and if the Camera admits of a picture of 

 that size being taken, a corresponding square should also be 

 marked on the screen. 



Now adjust the mirror so as to throw light into the instrument ; 

 if all has been accurately made and put up, the circle of light 

 will be just contained by its proper square : the apparatus is now 

 ready for use. 



The artist need not confine himself to a distance of 10 inches, 

 but that being the distance at which the magnifying power of a 

 Microscope is measured, it is the most convenient for general use, 

 as the amount of amplification will be always known without 

 calculation. 



No correction for actinism being required, the object should be 

 focused as for Microscopical examination, i. e. the best visual 

 focus possible should be obtained. The field should be equally 

 illuminated. With the lower powers this is very easy. Use the 

 flat mirror, and if the object slide is covered with paper, adjust it 

 so that a circle of light is thrown upon it, the object being in the 

 centre. If the slide be not covered a piece of paper or a card 

 placed upon it will enable you to see when the light is in the right 

 place. This adjustment will generally be found sufficient, and, if 

 not quite the thing, will require very little alteration. 



With powers above the half inch it will be generally necessary 

 to use the achromatic condenser, and here there will be some 

 difficulty in illuminating the field properly, and for reasons that 

 will readily occur to the Microscopist, one of which is, that when 

 a large instrument is put up in this way it is very difficult to reach 

 the mirror and look into the focusing screen at the same time, but 

 by placing the apparatus close to the side and end of the tube, as 

 previously directed, and sitting on a chair at the corner, it can be 

 accomplished with a little patience. 



The source of light (when that ^is the sun) must not be accu- 

 rately focused upon the object, as it will be reduced by the con- 

 denser to a mere point, but the condenser must be racked up or 

 down, (usually the latter) until the best light is obtained. 



It will be advantageous to use a focusing glass to examine the 

 image formed upon the focusing screen. Any positive eye-piece 



