582 APPENDIX. 



which issues from an object, the less is the loss which the light experiences in its 

 passage through so many media, partly by reflection at the surfaces, partly by 

 absorption in the interior. Two methods of using the microscope must here be 

 distinguished the examination of opaque and transparent objects. 



The first is the oldest, most simple, and natural method. It corresponds with 

 the manner in which we view objects with the naked eye by means of the light 

 diffused about them. Mere daylight usually suffices for this, if the magnifying 

 power is not very great ; but if the magnifying power is high, the light (which 

 in that case had best be artificial) is passed through a lens, or what is called 

 Selligue's prism*, and concentrated upon the object. The examination with 

 transparent light is very different. It is remarkable that no natural philosopher 

 has as yet presented us with a theory of this manner of seeing ; indeed, the 

 essential difference in the two modes of observation has not even been indicated 

 in any of the works on physics which I have read. It seldom occurs to us in 

 ordinary life except when observing air-bubbles or other irregularities, or slightly 

 cut designs in glass. The whole act of seeing here is founded on the different 

 reflection or absorption of the rays of light through unequally refracting and 

 unequally dense media in close proximity. The more powerfully refracting 

 or denser parts permit fewer rays of light to pass through them and arrive at 

 the eye, and appear, therefore, darker than the others. Indeed, it is very 

 possible that two substances lying near to each other, having equal densities and 

 equal refractive power, and therefore not to be recognised as different under the 

 microscope, may exhibit an evident difference through the circumstance of their 

 having a different polarising or depolarising effect upon light. The result, 

 therefore, would always depend on the greater or less quantity of light which 

 passes through the object from below. We must, however, take into consi- 

 deration that a different quantity of light is reflected according to the angle of 

 incidence and the direction of the rays of light coming from below. 



The usual contrivance in all microscopes is a mirror, moveable in all directions, 

 under the stage of the microscope. It is made either plane or concave, and the 

 latter is used in order that the pencil of light proceeding from it may exactly fill 

 up the opening in the 'stage ; and a greater quantity of light is also obtained in 

 this case. The best and most usual plan is to have both a plane and concave 

 mirror, turned with their backs to each other, in the same frame, so that they 

 may be alternated or changed at pleasure. If anything, the illumination by the 

 plane mirror is preferable ; the quantity of light is certainly not so great, but the 

 parallelism is decidedly of greater advantage for accuracy of observation. It 

 does not seem improbable that a distortion may take place in the image, through 

 the convergence of the rays in the concave mirror. My attention has often 

 been drawn to these phenomena ; but I confess that I know little about it, 

 as the opticians leave us perfectly in the dark on this point. According to 

 Wollaston, a converging lens may be appropriately used in the simple microscope 

 if a greater quantity of light is required. 



During the examination of delicate objects, it is, however, not unfrequently the 

 case that we are obliged to moderate the amount of light. The eye is too much 

 irritated by a strong light, when observing very transparent objects, to be able to 

 perceive slight or delicate differences, which are more readily perceived with less 

 light. For this purpose the plain mirror may be covered with a small piece of 

 white wood, ivory, or ebony, or it may be placed in such a manner that it sends 

 no rays upon the object. There is a peculiar contrivance in the stage of all well- 

 constructed microscopes, which serves as well to moderate the light as to allow it 

 to fall obliquely on the object. This contrivance consists of a disc, perforated 

 with holes of different sizes, which is attached under the stage in such a manner 

 that the light may be made to pass at will through one of the holes, or it may 

 even be excluded altogether. On placing this disc, which ought to be very easily 

 moveable, in such a manner that only a part of a hole meets at one side the 

 section of the stage, it will give us an oblique light. This contrivance (called 

 a diaphragm) is almost indispensable. We can only get rid of a great number of 

 illusions by a continual change of the light. An attentive observer will soon 



* A prism with two convex surfaces. 



