116 



THE MICROSCOPE AND ITS KEVELATIONS. 



Objectives, has been devised by Mr. James Smith. The Microscope 

 being in position for observation, the lamp should be placed either in the 

 front or at the side (as most convenient), so that its flame, turned edge- 

 ways to the stage, should be at a somewhat lower level, and at a distance 

 of about three inches. The bull's-eye should be placed between the 

 stage and the lamp, with its plane surface uppermost, and with its con- 

 vex surface a little above the stage. The light entering its convex sur- 

 face near the margin turned towards the lamp, falls on its plane surface 

 at an angle so oblique as to be almost totally reflected towards the oppo- 

 site margin of the convex surface, through which it passes to the object, 

 a little above the plane of the stage, on which it should cast a sharp and 

 brilliant wedge of light. The adjustment is best made by first placing a 

 slip of white card on the stage, and when this is well illuminated, substi- 

 tuting the object-slide for it; making the final adjustment while the 

 object is being viewed under the Microscope. No difficulty is experienced 

 in getting good results with powers of from 200 to 400 diameters; but 

 high powers require careful manipulation. Mr. Smith states, that he has 

 succeeded in illuminating by this simple method, minute objects (such as 



!EteS& 



Beck's Parabolic Speculum. 



Crouch's Adapter for Parobolic 

 Speculum. 



Diatoms and scales of Lepiaoptera], very brilliantly and clearly, upon a 

 dark field, under an immersion l-16th inch Objective. But he considers 

 that it answers better for objectives of moderate than of very wide angu- 

 lar aperture. * 



114. The Illumination of Opaque objects may be effected by reflection 

 as well as by refraction; and the most convenient as well as most efficient 

 instrument yet devised for this purpose is the Parabolic Speculum of 

 Mr. E. Beck (Fig. 88), which is attached to a spring-clip that fits upon 

 the Objectives (2 inch, 1-J inch, 1 inch, 2-3ds inch) to which ib is espe- 

 cially suited, and is slid up or down, or turned round its axis, when the 

 object has been broughtinto focus, until the most suitable illumination 

 has been obtained. The ordinary rays of diffused Daylight, which may 

 be considered as falling in a parallel direction on the Speculum turned 

 towards the window to receive them, are reflected upon a small object in 

 its focus, so as to illuminate it sufficiently brightly for most purposes; 



1 See " Journ. Roy. Microsc. Soc.," Vol. iii. (1880)) p. C98. 



