42 



As the axes of the milled-heads used in the fine and coarse adjustment are 

 parallel, and as in elevating and depressing the tube both turn in the same 

 direction, the change from one movement to the other is most convenient for 

 the hand of the operator. 



As regards details of construction we would refer to the article : "Em neuer Mikro- 

 skop-Oberbau", Zeitschrift fur Instrumentenkunde, vol. XVIII, 1898, pp. 129 133. 



The divisions on the millcd-head of the old type of micrometer move- 

 ment, as applied to our Stand IV enables one to read movements of 0. 5 m m 

 of the tube in the direction of the optical axis. One of the milled-heads of 

 BERGER's micrometer movement is also provided with a divided scale, each 

 division of which corresponds to a variation of 0.002 mm in the posi- 

 tion o f t h e tube. 



By means of above measurements of thickness may be made with a considerable 

 degree of accuracy. The upper and lower surfaces of the object are successively focussed 

 and the respective positions of the index read off on the divisions of the milled-head. 

 The difference between the two readings -- objectives of the dry series being used - 

 then represents the thickness of the corresponding stratum of air. 



If the refractive index of the object be known, its actual thickness may be approxim- 

 ately ascertained by means of this also. Inversely, the refractive index of an object 

 may also be approximately determined if its thickness be known. Assuming d = the 

 difference between the two readings, D = the actual thickness of the object, and ;/ = its 

 refractive index, then although approximately only -- the formula 



D = nd 

 holds good. 



In, for instance, estimating the thickness of a cover-glass, // mav be assumed to bo 

 equal to 1.5 without risk of appreciable error. 



In taking ""measurements of this kind it is best to use one of the higher dry lenses 

 and a medium ocular, using central illumination. 



With regard to the execution of more exact measurements of thickness by the aid 

 of the micrometer-screw see an article by S. CZAPSKI on "Die Bestimmung von Deckglas- 

 dicken an fertigen Praparaten" (Zeitschr. f. wiss. Mikrosk., vol. V, 1888, pp. 482484). 



t 



The draw-tube, which is possessed by all stands, with the exception of the 

 Stands IX, X, XI and the Hand-microscope admits of being increased or diminished ; 

 the existing tube-length (see pp. 8 and 9) can be read by means of a scale engraved 

 on the draw-tube, whereby the length of any apparatus -- nose-piece, 

 objective changer and the like inserted between tube and objec- 

 tive -- may be duly compensated for (see pp. 8 and 9). The lower end is 

 provided with the standard screw to take an objective of lower power, as, for 

 instance, the auxiliary objective used with the apertometer. This standard screw 



