252 PROGRESS IN MICROSCOPY 



devised so that such shift be directed at right angles to the shift origi- 

 nated by the diaphragm m. Thus is derived the absorption distribution 

 over the whole area of the object. 



Note 1. In devices similar to the one shown in Fig. 10.5, measure- 

 ments only deal with a small portion of the image to which is cor- 

 related an even smaller portion of the field object. Then the outlet 

 slit F of the monochromator may be imaged on to the specimen. 

 Even with a narrow sht providing fully-monochromatic light, the 

 width of the illuminated field is always adequate. In layouts similar 

 to the one shown in Fig. 10.1, the whole field object at E is included 

 in the photographic plate. If the slit of the monochromator is imaged 

 on the specimen, the slit is to have adequate width, covering the field 

 but this may give rise to wave-length changes throughout the image. 

 Conversely, the whole field is illuminated without any such change 

 if the slit is imaged in the condenser, duly adjusted to obtain the image 

 of /o (Fig. 10.1) on the specimen. Reducing the slit increases mono- 

 chromatism without lessening the width of the illuminated field. 



Note 2. In layouts similar to the one shown in Fig. 10. 1, the 

 specimen is illuminated throughout. The photo-analysed area receives 

 some stray light from all the illuminated points of the specimen. 

 Such light is due to the optical elements of the microscope and 

 diffraction-originated efi'ects. Every image element is a diffraction 

 disk spreading the light. Hence, luminous elements close to the studied 

 detail may convey light there owing to their diffraction rings. Such 

 effect can be very objectionable if the investigated object is close to 

 a much brighter detail. These drawbacks can be avoided by illumi- 

 nating only a very small area, instead of the whole specimen. 



Discounting design problems, setting effectively a diaphragm 

 against the object solves the difficulty. Since such an arrangement 

 is but seldom feasible and only when low magnification is involved, 

 the following method is to be preferred and applies to all cases. 



The light, originated from the monochromator, passes through 

 the microscope in reverse. A diaphragm, provided with a small 

 aperture, is set in the plane of the spectrum furnished by the mono- 

 chromator. This aperture is imaged on the specimen by the micro- 

 scope objective. The emerging light impinges against the condenser 

 and proceeds to the photoelectric cell. Figure 10.10 shows diagram- 

 matically the layout of the device. The diaphragm T with its small 

 aperture, is in llic position of the outlet slit of the monochromator. 



