168 PROGRESS IN MICROSCOPY 



number A^ of divisions of the drum T, tallying with /;' divisions of 

 the objective micrometer, is noted. Every division on the drum T 

 equates therefore 7i'iNn millimeters. The length to be measured is 

 assumed to be minute and in the field centre. 



4. MEASURING A THIN OBJECT BY TWO AXIAL SETTINGS 



Thin objects may be measured through a conventional microscope 

 by means of a previously-calibrated fine-focusing adjustment. The 

 screw of the latter usually comprises a datum-mark which is moved 

 opposite the graduation. Such graduation can be calibrated as follows: 

 let us use as calibration reference an optically-tested cover-glass the 

 thickness e of which is ascertained. This cover-glass is semi-aluminized 

 on both faces, each of which has an optical density approaching unity, 

 i.e. 2 in the aggregate, approximately. Therefore, the cover will only 

 let 1/100 of the incident light pass through, which is adequate. Next, 

 both faces are scratched. These scratches, plainly visible in a suitable 

 contrast, serve as reference-marks to set the object-cover faces. There- 

 upon the cover-slip is set in the microscope stage (the cover-slip being 

 secured to the object slide, if required) and both faces of the cover- 

 glass are sighted seriatim by means of the fine adjustment, using the 

 scratches as reference-marks. Since p divisions of the fine-adjustment 

 screw correspond to the thickness e in millimetres of the cover-glass, 

 a one-division shift of the screw equates an ejp thickness (in mm) 

 in a medium of index ;?, index of the cover-slip, and would correl- 

 ate an ejnp shift (in mm) in air and n'ejup (in mm) in a medium of n 

 index. 



In order to obtain the actual length, the vertical length comprised 

 in a medium of index n' is to be multiplied by //'. The presence of 

 a cover-glass having a different index does not alter measurements. 



5. MEASURING A LENGTH WHOSE ENDS DO NOT LIE IN THE SAME 



FOCUSING PLANE 



The length to be measured is A^A. (Fig. 5.4) but, as neither A^ 

 nor Ai lie in the same front plane, focusing must be altered when 

 changing over from A^^ to A.,. Once A., is focused, the location of the 

 image A'.^ is ascertained by means of the eyepiece micrometer spider-web. 

 Next, in the case shown in Fig. 5.4, in order to focus A^, the microscope 

 is tilted upwards slightly to the extent of f . Things work out as if A<> 

 remained in focus and that the setting on Bi (projection of /ii), was 



