280 Transactions of the Society. 



scope is arrived at by multiplying together the magnifying powers 

 of these separate components. 



Method 3. — The angular aperture of the transmitted beam is 

 measured (a) as it enters the aperture of the objective and (b) as it 

 leaves the aperture of the eye-lens. The magnification is arrived 

 at by dividing the first measurement by the second. 



Method 4. — By the exploitation of a fiduciary phenomenon, 

 which is obtained by the aid of a diffraction grating, an observa- 

 tion is made which furnishes the distance between the lines of a 

 micrometrical ruling as imaged upon the retina by the aid of the 

 Microscope or other magnifying system. This observation made, 

 a series of paired lines is viewed through the grating by the 

 unaided eye from a distance of 10 in. The observer — taking to 

 aid the fiduciary phenomenon before referred to — now seeks out 

 that pair of rulings which furnishes upon his retina an image 

 exactly similar to that obtained with the assistance of the magni- 

 fying system employed in the first observation. 



The magnification is now obtained by measuring the interval 

 between the pair of object lines which complies with this condition, 

 and by dividing this measurement by the interval between the 

 lines of the micrometrical scale. 



Measurement of the Magnification by bringing the Pencils 

 of Parallel Pays which emerge from the Microscope to 

 focus ufon a Measuring Scale, and reading off the 

 Dimensions of the Image thus formed by means of an 

 Eye-piece. 



The image I now throw on the screen (reproduced in Fig. 54) 

 exhibits the construction of the simple piece of optical apparatus 

 — we may denote it the eikonometer — which allows of the dimen- 

 sions of the microscopic image being read off at a glance. 



At A is disposed a plano-convex lens, which, like the refractive 

 system of the eye, brings to focus the pencils of parallel rays 

 which emerge from the eye-lens of the Microscope. 



In the case of the instrument which I have placed upon the 

 table, a focal length of 1 in. has been given to the focussing lens. 



Such a lens furnishes upon its principal focal plane an image, 

 whose dimensions are ten-fold smaller than those of the imao;e 

 which is projected outwards from the retina to the conventional 

 distance of 10 in. 



In the principal focal plane, just spoken of, is placed a micro- 

 metrical scale ruled in tenths of millimetres. These have, for the 

 purpose of the measurement of the image furnished by the focussing 

 lens, the same value as millimetre divisions, applied to the image 

 projected outwards from the eye to a distance of 10 in. 



