72 



THE AMERICAN MONTHLY 



[April, 



scope for the ordinary camera-lens. 

 In order to make a perfectly light- 

 tight connection between the camera 

 and the microscope, I fasten over the 

 opening of the camera a sheet of thick 

 rubber having a hole cut in it some- 

 what smaller than the body of the mi- 

 croscope. Then, when the body of the 

 microscope is thrust through the open- 

 ing, the rubber closes tightly around 

 it and makes a perfect joining.The eye- 

 piece can then be inserted from the 

 inside of the camera, if desirable. In 

 photographing without the eye-piece 

 there is apt to be a bright spot in the 

 centre of the field of view, due to ir- 

 radiation from the interior of the body 

 the microscope. The use of the eye- 

 piece entirely obviates this difficulty, 

 and, at the same time secures greater 

 magnification; but, of course, the 

 field is not so brilliantly lighted. If, 

 however, it is desirable to dispense 

 with the eye-piece, the bright spot can 

 can be avoided by lining the interior 

 of the tube with paper which has a 

 dead-black surface. 



As it is desirable that the object 

 shall be as well illuminated as possi- 

 ble, it is best to take the light direct 

 instead of reflecting it upon the ob- 

 ject by means of the mirror. It is 

 also best to use two condensers, a 

 large one for collecting as many rays 

 as possible and concentrating them 

 upon a smaller condenser, which, in 

 turn, concentrates them upon the ob- 

 ject. An achromatic condenser may 

 be advantageously substituted for the 

 latter. 



As some who desire to experiment 

 in this line may desire a starting point 

 as regards the matter of exposure, I 

 would say that with the light of a stu- 

 dent lamp, and using a single con- 

 denser, I have found that from i ^ to 

 2]/^ minutes with a 2-inch, 2)^ to 5 

 with a i-inch, and 4 to 7 minutes with 

 a ^-inch objective, are about the 

 proper times when the A eye-piece is 

 in, and using what are known as Car- 

 butt's rapid (B) plates, No. 468. 

 When the eye-piece is not used about 

 one-half of that time is required. Of 



course the time of exposure will vary 

 somewhat, according to the density 

 or transparency of the object, and, if 

 stained, according to the character of 

 the coloring matter. 



Apparent Motions of Objects. 



BY PROF. F. C. VAN DYCK. 



Objects viewed through the micro- 

 scope seem to move when the posi- 

 tion of the mirror is slightly changed. 



This fact is doubtless familiar to 

 most microscopists, but has not, so far 

 as I know, been discussed in its opti- 

 cal bearings. 



The phenomenon is easily observed 

 by using nearly parallel rays to illu- 

 minate the object, and placing the 

 mirror approximately central under 

 the stage. If daylight is used, set the 

 microscope at a considerable distance 

 from the window, and use the plane- 

 mirror. If lamplight -is used, set the 

 lamp at the focus of the concave 

 mirror, or use a lens to make the rays 

 parallel and reflect them from the 

 plane-mirror. 



If the object be so thin as to be 

 sensibly in one plane, it will maintain 

 its location in the field whatever 

 change be made in the position of 

 the mirror, so long as it is accurately 

 focussed. But if the tube of the mi- 

 croscope be raised or lowered, so as 

 to throw the object slightly out of 

 focus, a shifting of the mirror on its 

 bearings will cause an apparent mo- 

 tion of the object to one side or the 

 other. 



If an object of considerable thick- 

 ness be used and the focus obtained 

 for a central plane, rocking the mir- 

 ror will cause the lower parts of the 

 object to move to one side, while the 

 upper parts move to the other side. 

 I have an insect's foot with claws, 

 which, treated in this way, seems to 

 work the claws like scissors. Minute 

 details of an object may be made to 

 disappear under spots on the cover- 

 glass, and various similar effects can 

 be produced. 



