608 SUMMARY OF OURRKNT RESEARCHES RELATING TO 



colour into a second colour, but without saturating it, with the Micro- 

 scojje we see an outer ring, which is the result of the two colours, and 

 a central part of the primitive shades. 



Apparent Motions of Objects.* — Prof. F. C. Van Dyck considers 

 that the familiar fact that objects viewed through the Microscope seem 

 to move when the position of the mirror is slightly changed, has not 

 been discussed in its optical bearings. 



" The phenomenon is easily observed by using nearly parallel 

 rays to illuminate 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 jjlane 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 Microscope 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 ajiparent motion of the object to one side or the other. 



If an object of considerable thickness be used and the focus 

 obtained for a central j)lane, rocking the mirror will cause the lower 

 parts of the object to move to one side, while the uj^per 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. 



Let us suppose that the illumination is received from the left of 

 the observer, and that a micrometer is inserted in the eye-j)iece to 

 facilitate observation. Take three points A, B, and C, in the optical 

 axis, A beyond the focus of the objective, B at the focus, and C a 

 little above the focal plane. Suppose a pencil sent from the mirror 

 along the axis, jiassing A, B, and C, and the centre of the objective. 

 The images of A, B, and C, will fall with their centres on the axis. 

 If the edge of the mirror toward the observer's right be tilted up, the 

 point A, beyond the focus, will appear to be displaced toward the right 

 of the field of view, the point B will remain stationary, and C which 

 is above the plane focussed upon, will move toward the left. Now it 

 can be shown that if the spherical aberration of an objective could be 

 corrected for a series of points and their images, all the images must 

 remain stationary. 



The necessity for correction consists essentially in the fact that 

 the margins of lenses with spherical surfaces are too strong relatively 

 to their centres. Hence, with an uncorrected lens, the image of the 

 point B, made by the central portion of the lens, would fall on the 

 axis ; but an image of the same point, produced by rays entering the 

 left-hand margin, would fall to the left of the axis, as well as nearer 

 to tlie lens. The essence of correction is to relatively weaken the 



* Amer. Moii. Micr. Jourii., iii, (1882) pp. 72-3. 



