INSTRUMENT CLASSIFICATION AND APPLICATIONS 



point depend on the mean path difference 

 over a surrounding area equal to the out-of- 

 focus disc corresponding to the axial dis- 

 tance between the two image.s. This effect 

 would not be serious if this distance could 

 be made large, but in practice it is limited 

 by considerations of aberrations of the back 

 focal plane of the objective, so path differ- 

 ences measured by this instrument should 

 be treated with some reserve. 



To avoid this effect another type of po- 

 larizing microscope due to Dyson (4) (Figure 

 3) employs two thick plates of Iceland spar 

 cut parallel with the axis, one above and one 

 below the object, the working distance of the 

 objective being increased by a suitable op- 

 tical system. The out -of -focus disc is now 

 strongly astigmatic. As both sphtting and 

 recombination take place in the object space 

 the limitations on the size of the disc are 

 removed and it can be made larger than the 

 field of view, which considerably reduces its 

 effect on the micertainty of measurement. 



As both these instruments make use of 

 beams which are polarized in mutually 

 perpendicular directions, polarimetric means 

 may be used to measure the path difference 

 with considerable accuracy. 



The effect of the out-of-focus disc may be 

 removed entirely by another instrument due 

 to Dyson (1) (Figure 4). Two glass plates, 

 one or two millimeters thick and partially 

 silvered on both surfaces, replace the Iceland 

 spar plates shown in Figure 3. The ray paths 

 are similar to those in a Jamin interferom- 

 eter and the out-of-focus disc can be made 





XceloncJ span 

 plat'G.s c:u+ 

 parol lei Wii^t^,^- 



Opl'ic axis 



UJ 



Objective 



Qloss block w/itl-i 

 upp*in suntcJCe 

 Silvered 

 Object 

 >/2 Plate 



Fig. 3. Dyson's interference microscope with 

 crystal plates. 



Fig. 4. Dyson's interference microscope with 

 half -silvered glass plates. 



many times the diameter of the field. As the 

 reflecting system introduces a small central 

 stop, a dark center, which can be made 

 larger than the field, occupies the center of 

 the out-of-focus disc, which therefore has no 

 influence on the measurement, which is quite 

 explicit. 



The background path difference and the 

 number of fringes in the field are both varied 

 by making the two plates slightly wedge- 

 shaped. The number of fringes is varied from 

 zero (uniform field) to a maximum by rota- 

 tion of one plate about the microscope axis, 

 and the path difference by translation of the 

 other plate by means of a micrometer screw. 

 By means of a photometer eyepiece measure- 

 ments of path difference may be made with 

 an accuracy of X/300. The highest powers of 

 objectives can be used and the aperture of 

 illumination is unrestricted. 



As the separation of the images is so large, 

 the leveling adjustment of the plates is quite 

 critical, and the instrument is therefore more 

 difficult to adjust than the others in this 

 class. This is the price paid for unambiguous 

 measurement and freedom from optical arti- 

 facts. 



Instruments of Class III. Instruments 

 of this class are usually known as "shearing" 

 microscopes. An early instrument due to 

 Lebedev (5) (Figure 5) made use of two 

 Iceland spar plates cut at 45° to the optic 

 axis, with an intervening half-wave plate to 

 interchange the planes of polarization. Two 

 images are thus seen, sheared laterally with 

 respect to each other, one being somewhat 



415 



