ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 369 



The Screen. — The screen consists of a disk of glass on which are 

 photographed rulings forming a reticule of five divisions to the square. 

 It is thus apparent that, by adjusting the screen at various distances, a 

 micrometer of various known dimensions is obtained. In the perpen- 

 dicular position, the screen is reflected by the illuminating mirror through 

 the condenser, by which an image of the reticule is formed on the object. 

 In the horizontal position, the micrometric image is transmitted direct 

 through the condenser, and an image of the reticule is formed on the 

 object. The screen may be readily swung in or out of the field as desired. 



The Direct Method. — The Microscope is placed horizontally. The 

 screen is centred by focusing with the condenser a small spot in the 

 centre of the ground glass. The grouud glass is now removed, and the 

 screen is placed in the carrier in its stead. The object to be measured 

 is focused in the ordinary way, and then the scale is focused by the 

 condenser, an image of the scale is thrown on the object, the scale 

 which has been previously calibrated on the rails is read off, and the 

 size of the object determined. 



The Indirect Method. — This is used when it is necessary to have the 

 Microscope in an upright position. The same applies here as regards 

 the scale, only in this method the plane mirror is used to reflect the 

 parallel rays up through the condenser. First centre the light with the 

 mirror (plane side), focus the stage micrometer as before, place your 

 screen in situ, and focus it on to the stage micrometer, compare with it, 

 and calibrate your rails accordingly. 



Advantages over other Micrometers. — 1. The micrometer is easily 

 placed in or out of the field whilst examining an object. 2. The whole 

 field is divided into squares. 3. It can readily be adjusted to various 

 known measurements by simply sliding the carrier on the calibrated 

 rails. 4. It has advantage over the eye-piece micrometer in so much as 

 it is not always in the field of vision, and moreover it has a greater 

 range of measurements. 5. It measures areas instead of lines. 6. By 

 using the camera lucida. the object mav not onlv be easilv drawn, but 

 accurately measured. 



This ghost micrometer is not new and has been repeatedly re- 

 invented : e.g. by Goring circa 1820 ; Eovston Pigott, circa 1*70, 

 A. E. Wright, 1890, and others. 



(4) Photomicrography. 



- Method of Estimating the Exposure in Photomicrography, with 

 Axial Cone Illumination.* — Duncan J. Reid remarked that it is 

 generally admitted that to obtain good results in photomicrography, it 

 is necessary that the exposure should be correct. One important feature 

 of this method is that it enables one to record the conditions under 

 which a photograph had been taken, and that with such accuracy as to 

 make it possible to repeat it on any future occasion. It is based on the 

 method of calculating the " working aperture," by the measurement of 

 the Ramsden disk of the eye-piece, described by Sir A. E. Wright in his 

 book on the " Principles of the Microscope." It presupposes carefully 

 centred axial cone illumination, from a source of light at a fixed distance, 



* Journ. Quekett Micr. Club, 1909, pp. 486-9. 

 Jum loth, 1910 2 C 



