1893.] THE MICKOSCOPE. 47 



times, and so on. The camera lucida is not absolutely essential, 

 as the observer may view the stage-micrometer in the microscope 

 with, say, his right eye, and holding the rule 10 inches from his 

 left, shift it about until the image of the stage-micrometer is su- 

 perposed on it as before ; but this method is inferior both in com- 

 fort and accuracy to that described. — English Mechanic. 



Objectives with Correction Collars. — In 1S29 Amici first 

 noticed that high-power objectives, which gave a perfectly clean 

 image when objects, not covered with a glass, were examined, 

 did not give so good a one when the object was covered, and 

 that the clearness of the image increased and diminished accord- 

 ing to the thickness of the cover-glass. To remedy this defect, 

 which resulted from spherical aberration, Amici constructed his 

 objectives in such a manner that they could all be used with 

 cover-glasses of a definite thickness. 



In 1837 the celebrated English optician, Ross, though ignorant 

 of the discovery of Amici. made the same observation, and. to 

 remedy the defect, he invented correction objectives. 



In this kind of objective the two upper lenses occupv an invari- 

 able position with regard to one another. They are fixed in a 

 movable tube, and can be made to recede from or approach the 

 lower part, which is fixed, and carries the single or double 

 frontal. By turning a ring the upper lenses rise or fall, and a 

 coiled spiral spring regulates the small inequalities of the screw, 

 and above all prevents the ''backlash," which is always pro- 

 duced when the sense of motion is changed. The firm of Carl 

 Zeiss engrave on their correction objectives a very practical 

 series of figures. This series of figures, which extends from ten 

 to twenty-five (or less), indicates at once the position to be taken 

 for any given thickness of cover-glass. If we have, for example, 

 to study a preparation covered with a cover-glass 1^ tenths of a 

 millimetre thick, the division 15 is placed against the index, and 

 we shall have the best position of the lenses in which to study the 

 preparation in question with axial light. — From The Microscope 

 by Van Heitrck. 



' Mounting Medium. — One of the best preparers of diatoms 

 uses for a medium a mixture of Canada balsam and mono- 

 bromide of naphthaline The refractive index of this mixture is 

 much higher than that of balsam alone, while the image is clear 

 and brilliant. 



CORRESPONDENCE. 



Replies to Questions. — The following questions, which were 

 printed on page 32, have been answered by Arthur M. Edwards. 

 M. D., Newark, N. J. 



