45 



2. BERGER's micrometer movement far surpasses the old in delicacy, is also 

 free from excessive one-sided weight, and is provided with safety appliances 

 which almost entirely exclude the chances of accidental injury. 



3. The most natural place for grasping the microscope for purposes of removal 

 is by the column supporting the upper body. By the new form of construction we 

 were enabled to convert the upper portion of this column into a convenient handle, while 

 an attempt to lift a microscope of the old type in the same manner would most likely 

 have resulted in damage to the prismatic movement as well as to the micrometer-screw. 



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

 ment, as applied to our large and medium stands, enables one to read move- 

 ments of 0.005 mm 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 position of the 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 focused 

 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 an object in air. 



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

 mately ascertained by means of this also. In the reverse way, 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 approximate! v only -- the formula 



D = n d. 

 holds good. 



In, for instance, estimating the thickness of a cover-glass, n may be assumed to be 

 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 eye-piece, 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 Dcckglas- 

 dicken an fertigen Pra'paraten" (Zeitschr. f. wiss. Mikrosk. vol. V, 1888, pp. 482 484). 



, Sena. 



