REFLECTED-LIGHT MICROSCOPY 149 



in Fig. 4.29(a). The light is originated from the lamp S (opal or 

 diffusing); after being reflected on the 45''-slanted semi-reflective 

 plate G it passes through the polariscope Q and is reflected by the 

 object F and reflected again on to the magnifying glass O and the 

 ocular diaphragm T behind which is the observer's eye. This ocular 

 diaphragm is in the focus of the magnifying-glass O. In order to focus, 

 the object P is shifted in relati€)n to the instrument whose elements 

 are all secured together. The polaroid element iP^ is so arranged as 

 to emit a vibration at right angles to the incidence plane on G. The 

 second polaroid element fPo is crossed in relation to ^i. Hence, 

 were it not for the presence of Q no light reaches F. The polariscope Q 

 is not of the Savart type. It consists of two quartz plates, cut 45° 

 from the optical axis and whose axes are in the same plane and at 

 right angles. A Cellophane half-wave plate is cemented between the 

 two plates, 45° from the quartz axes. A single passage of a Hght-beam 

 through this polariscope gives rise to on-infinity fringes which 

 are parallel and equidistant straight lines (the on-infinity Savart fringes 

 are only approximately straight lines). Two passages of the light beam 

 cause the on-infinity fringes to overspread completely and the tint 

 is flat. There is no need for any diaphragming and the lamp is ac- 

 cordingly used without a collimator. 



According to the thickness of the polariscope Q either the full- 

 duplication or the differential method may be used (Chap. Ill, § 5). 

 The same device is also applicable to a low magnifying power micro- 

 scope (Fig. 4.29(b)). In this case, the system G, similar to that shown 

 in Figs. 4.23, 4.24 and 4.25 is inserted between the objective Oi and 

 the polariscope Q. It should be noted that the layouts shown in 

 Figs. 4.29(a) and (b) are completely devoid of stray light : any light-beam 

 which has not passed through Q is occluded since f?i and ^o are 

 crossed. The light reflected by the underface of Q is also occluded 

 since, if Q is set in the proper direction, the light passing through Q 

 twice and which is reflected against its underface exhibits on-infinity 

 fringes that are approximately zero at T. Therefore, there is no 

 reflected stray light in the eyepiece-lens plane. Only the light reflec- 

 ted back by the object F reaches the observer, as shown in Chap- 

 ter III. 



NomarsUVs device (Fig. 4.30.) 



A Wollaston prism W, set in the objective's focus, is used in this 

 instrument. The light originated from the vertical illuminator is 



