8 THE MICROSCOPE. [January, 



against the rim of the mirror, and solder a small hook of brass 

 or copper wire to either end of the clasp. This is to be 

 fastened across the concave surface, diametrically opposite to 

 the fork which holds the mirror, leaving the plane surface free to 

 reflect the light. The centre of a rather stout wire eight inches 

 long is to be fastened by clamp or string to the lower part of the 

 mirror-fork in a direction opposite to the first clasp. Make a 

 small hook on either end of the wire. Let the mirror, fork, base- 

 piece and clasps attached be clamped in an upright position at 

 such a heigrht that the centre of the mirror shall be on a line with 

 the optical axis of the other parts. 



Now construct a wooden frame that shall hold a roller as fol- 

 lows : Let it be eight inches long and one inch wide and consist 

 of a base-piece with an upright two inches long fastened to 

 either end. The wooden roller is one inch in diameter, long 

 enough to reach between the uprights, and arranged to revolve on 

 its axis between the ends. By means of a screw fasten the frame 

 at its centre to the base-board under the camera-cone in a handy 

 position, so that it may be turned in a horizontal direction. Let 

 a string run from the hook on one end of the first clasp through 

 a small screw-eye under the mirror, around the roller twice, back 

 through a screw-eye, and attach to the other end of the clasp. 

 From either end of the frame that supports the roller extend a 

 string to the end of the stout wire fastened to the mirror-fork. 

 It is evident that any position of the mirror may now be obtained 

 by manipulating the wooden frame and roller. 



But the mirror attached to the microscope, though farless con- 

 venient than the above arrangement, may be made to answer the 

 purpose, and, although a little intractable at first, one soon learns 

 to guide the beam aright. The beam must pass through the 

 optical axis of condenser, objective, microscope tube, and camera 

 to produce the right results. If eccentric, the image will be im- 

 properly illuminated, or distorted. If no condenser is employed, 

 a diaphragm should be substituted. The size of orifice can be 

 determined by noting the best result of definition, especially in 

 the periphery of the illuminated circle upon the ground glass. 

 In using the condenser, rack it back and forth from the stage 

 until the best result is obtained, or until the field is equally 

 lighted in all parts, then shut off the extra light with a proper 

 diaphragm, or preferably by using the iris diaphragm if it has 

 one attached, until the definition is the best that can be attained. 

 A little experience will enable one to arrange the condenser in 

 the best position for obtaining a good, sharp negative. 



[ To be continued.] 



