352 



MICROSCOPE. 



field of fourteen inches. In viewing large ob- 

 jects for which great perfection is not required, 

 we find it advantageous to interpose a ground- 

 glass at about half or three quarters of an inch 

 distance beneath them ; this is easily adapted to 

 the under side of the stage. It serves to pro- 

 duce an extremely equable diffusion of the 

 light over a large field, and to deaden the glare 

 which is occasioned by the direct admission of 

 so large a quantity ; but, if applied to objects 

 which require to be seen with great distinct- 

 ness, it will be found to produce a kind of fog 

 which seriously impairs the power of the micro- 

 scope; and with objects of any difficulty it is 

 quite inadmissible. In viewing objects, how- 

 ever, of the largest size that the microscope can 

 receive, by a good diffused daylight, the ground- 

 glass is never required. A bright cloud, oppo- 

 site to the sun, is then the best source of illu- 

 mination. For the purpose of effecting these 

 and other adjustments of the mirror, we have 

 found it convenient to have the tube which 

 carries it sprung, so as to slide rather loosely on 

 the stem, and to secure it in any particular 

 situation by means of a large milled head which 

 screws on one end of the tube, and clamps it 

 upon the stem. 



The plane mirror, with a condensing lens 

 between it and the stage, is preferred by many 

 to a concave minor; and for certain objects it 

 is, without doubt, superior. For ordinary use, 

 however, we prefer the concave mirror, for the 

 following reasons : Its effects are obtained by 

 a single adjustment ; whereas, in the use of the 

 plane mirror and lens, two adjustments are re- 

 quired : and if (as we shall presently state to 

 be often desirable) the mirror be thrown quite 

 out of the axis of the optical part of the instru- 

 ment, it is difficult to adjust the condenser 

 with correctness. Further, in order to obtain 

 light enough for a field such as we have men- 

 tioned, the condensing lens must be nearly 

 three inches in diameter, and thus becomes 

 Very cumbrous and inconvenient. For objects 

 of a high class the concave mirror should of 

 course not be employed ; but for these the 

 common condenser is by no means adapted, 

 and must be put aside for a superior one, such 

 as we shall presently describe. The plane mir- 

 ror and condenser enable the observer, how- 

 ever, to obtain an additional variety of illumi- 

 nation, which is often advantageous ; and by a 

 simple modification, proposed by Mr. Varley, 

 the light of a bright cloud may be artificially 

 imitated by them. The means of doing this 

 consist in covering the surface of the plane 

 mirror with carbonate of soda or pounded glass, 

 by which the direct solar rays are reflected very 

 much as by a white cloud. We have also seen 

 a plaster of Paris mirror employed for the same 

 puipose, and with good effect, where, on ac- 

 count of the transparency of the object, it was 

 necessary to reduce the amount of light sent 

 through it, without interposing any screen that 

 should produce indistinctness. 



It is often very desirable to throw the centre 

 of the mirror a good deal to one side of the 

 axis of the body of the microscope, so that the 

 reflected rays may fall very obliquely upon the 



object, and cause its prominences and depres- 

 sions to exhibit shadows of much greater depth 

 than can ever be seen with more direct light. 

 No microscope, in which there is not a provision 

 for this movement, can be regarded as having 

 its resources properly developed ; and we have 

 seldom seen one which comes up to our ideas 

 of the degree in which it should be permitted. 

 In general, the mirror-frame is immediately 

 fixed to the sprung tube which carries it; and 

 thus it can only be turned out of the axis at an 

 angle which will evidently interfere with its use. 

 In our own microscope, the mirror-frame is 

 connected with the tube by a stalk of an inch 

 in length, so that the centre of the mirror is 

 three inches from the stem. This, of course, 

 involves a lengthening of the stage, and of the 

 arm which carries the body, in order that the 

 centre of the apertures of all three may be in 

 the same line; but the disadvantage hence 

 resulting is easily avoided by increasing the 

 strength of these parts. The variety of illumi- 

 nation which may be given by a mirror fitted 

 in the manner we have described, is very great ; 

 and some very curious and unexpected pheno- 

 mena are not unfrequently disclosed by means 

 of it. For example, the field may be rendered 

 almost dark, by turning the centre of the mirror 

 considerably out of the axis, so that none of the 

 rays reflected by it pass up the body of the 

 microscope; whilst objects of great delicacy 

 will frequently appear brilliantly illuminated, 

 on account of their retention of a part of the 

 light which is passing obliquely through them. 

 In this manner we have often been enabled to 

 see an immense number of the minutest ani- 

 malcules (monads) rapidly moving through 

 water, in which, with direct light, none but the 

 larger ones could be distinguished ; and the 

 interest of the spectacle is heightened by the 

 phosphorescent glow which the animalcules 

 appear to have when thus illuminated.* A 

 similar effect may be produced without the 

 use of the concave mirror, by causing a direct 

 pencil of rays from a lamp or candle to be 

 tlirown very obliquely upon the object by 

 means of a condensing lens. 



In the examination of many delicate objects 

 with high powers, direct light will often be 

 found more advantageous than reflected. This 

 may be obtained with facility, by placing a 

 lamp or wax-candle behind the stage, the mirror 

 being turned out of the way. In the day-time 

 the direct light of a bright cloud will often be 

 found to produce an extremely beautiful effect; 

 but in order to attain this without an inconve- 

 nient position of the head, the microscope stand 

 must be elevated to such a height, that, when 

 the stem is horizontal, or even inclined in a 

 position contrary to the usual one, the eye-piece 

 may be at a convenient height for the eye. The 

 different modes of illumination best suited to 

 different objects can only be found out by ex- 

 perience, since achromatic objectives vary in 

 their relative effects with each. 



* This method of viewing objects was publicly 

 noticed, a few years since, as a new discovery; it 

 had long, however, been familiar to ourselves, 

 and, we believe, to most other scientific observers. 



