DETERMINATION OF MAGNIFYING POWER. 183 



among them the very beautiful Grammatophora subtilissima and 

 the Hyalodiscus subtilis ; the latter being of discoid form, and 

 having markings which radiate in all directions, very much like 

 those of an engine-turned watch. To these may be added the 

 Surirella gemma, of which the structure has been elucidated by 

 M. Hartnack. "When the valve of this species (Fig. 91) is examined 

 under very oblique illumination in the direction of its length, and 

 with an amplification sufficient for the resolution of the marginal 

 lines of the Grammatophora subtilissima, a series of strongly 

 marked transverse ribs is seen, the spaces between which are 

 divided by parallel lines, as shown in the upper half of the figure. 

 When, on the other hand, the valve is illuminated in a direction 

 transverse to its axis, the spaces between the ribs seem to be 

 divided by lines which cut each other very obliquely, so as to form 

 elongated hexagons by their intersection, as shown in the lower 

 half of the figure. A portion more highly magnified under the 

 same illumination shows the hexagons separated by the zigzag 

 lines a a, which are connected by the transverse lines b b. 



133. Determination of Magnifying Power. — The last subject to 

 be here adverted-to is the mode of estimating the magnifying power 

 of Microscopes, or, in other words, the number of times that any 

 object is magnified. This will of course depend upon a comparison 

 of the real size of the Object with the apparent size of the Image ; 

 but our estimate of the latter will depend upon the distance at 

 which we assume it to be seen ; since, if it be projected at different 

 distances from the Eye, it will present very different dimensions. 

 Opticians generally, however, have agreed to consider ten inches as 

 the standard of comparison ; and when, therefore, an object is said 

 to be magnified 100 diameters, it is meant that its visual image 

 projected at 10 inches from the Eye (as when thrown down by the 

 Camera Lucida, § 71, upon a surface at that distance beneath), has 

 100 times the actual dimensions of the object. The measurement 

 of the magnifying power of Simple or Compound Microscopes by 

 this standard is attended with no difficulty. All that is required 

 is a Stage-Micrometer accurately divided to a small fraction of an 

 inch (the l-100th will answer very well for low powers, the 

 l-1000th for high), and a common foot-rule divided to tenths of 

 an inch. The Micrometer being adjusted to the focus of the 

 Objective, the rule is held parallel with it at the distance of ten 

 inches from the eye. If the second eye be then opened whilst the 

 other is looking through the Microscope, the circle of light included 

 within the field of view crossed by the bines of the Micrometer will be 

 seen faintly projected upon the rule ; and it will be very easy to 

 mark upon the latter the apparent distances of the divisions on the 

 Micrometer, and thence to ascertain the magnifying power. Thus, 

 supposing each of the divisions of l-100th of an inch to correspond 

 with li inch upon the rule, the linear magnifying power is 150 

 diameters ; if it correspond with half an inch, the magnifying 



