PROJECTION MICKOSCOPY 



In Fig. 16b, the specimen can be thought of 

 as being replaced by two fihii images, ob- 

 tained by projecting the various points with 

 each of the eye pupils as a projecting center. 

 Fig. 16c shows that the same images can also 

 be the result of two equal objects which are 

 M times smaller (and thus have the dimen- 

 sion of the real specimen under study). As 

 the two objects are exactly identical, the 

 two film images are the projections of one 

 object, with the projection center shifted 

 over a distance As (Fig. 16d). Fig. 16e shows 

 that instead of moving the projection center, 

 the specimen itself may be shifted over a 

 distance Aa = As. The geometrical condition 

 for making stereographs ^\i\\ be: 



As = Sa = ae/l 



{2) 



in which a is the source to specimen distance. 

 In general, the value of a cannot easily be 

 measured, especially for high magnification. 

 The source to film distance 6, however, is in 

 most cases a constant of the instrument. So, 

 introducing the magnification on the film, or 

 primary magnification 



equation (2) can be transformed to 



il/pAs = MjAa = he/l (4) 



in which the second member is a constant. 



The antecedent is the image displacement 

 on the film or screen. So to satisfy this con- 

 dition, neither the magnification nor the 

 specimen displacement need be known. If, 

 however, Aa is known, by means of a cali- 

 brated specimen shift, Mp can be calculated 

 from the relative displacement on a stereo- 

 scopic pair of negatives. As this method of 

 measuring the magnification is the only gen- 

 erally reUable one, all projection microscopes 

 should be provided with a calibrated speci- 

 men shift. 



The source-to-specimen distance is related 

 to the magnification by the following equa- 

 tion: 



a = l/M 



(5) 



Mp = 



b/a 



This relation gives the order of magnitude 

 of a. As the viewing distance I is not very 

 critical, it is not necessary to know a accu- 

 rately. Note that in equation (5) M is the 

 final magnification. Inserting I = 300 mm 



(3) and M = 1000 gives a = 0.3 mm. So this 



Fig. 17. Depth distortion in contact stereomicroscopy. 



671 



