629 



MICROSCOPE. 



MICROSCOPE. 



It was at one time hoped, as the precious stones are more refractive 

 than glass, and as the increased refractive power is unaccompanied by 



a correspondent increase in chromatic dispersion, that they would 

 furnish valuable materials for lenses, inasmuch as the refractions would 

 be accomplished by shallower curves, and consequently with diminished 

 spherical aberration. But these hopes were disappointed : everything 

 that ingenuity and perseverance could accomplish was tried by Mr. 

 Varley and Mr. Pritchard, under the patronage of Dr. Goring. It 

 appeared, however, that the great reflective power, the doubly-refract- 

 ing property, the colour, and the heterogeneous structure Of the jewels 

 which were tried, much more than counterbalanced the benefits 

 arising from their greater refractive power, and left no doubt of the 

 superiority of skilfully made gloss doublets and triplets. The idea is 

 now, in fact, abandoned ; and the same remark is applicable to the 

 attempts at constructing fluid lenses, and to the projects for giving to 

 glass other than spherical surfaces. 



By the term timple microscope is meant one in which the object is 

 viewed directly through a lens or combination of lenses, just as we 

 have supposed an arrow or an insect to be viewed through a glass held 

 in the hand. When, however, the magnifying power of the glass is 

 considerable, in other words, when its focal length is very short, and 

 its proper distance from its object of consequence equally short, it 

 requires to be placed at that proper distance with great precision : it 

 cannot therefore be held with sufficient accuracy and steadiness by 

 the unassisted hand, but must be mounted in a frame having a rack 

 or screw to move it towards or from another frame or stage which 

 holds the object. It is then called a microscope, and it is furnished, 

 according to circumstances, with lenses and mirrors to collect and 

 reflect the light upon the object, and with other conveniences which 

 will now be described. 



One of the best forms of a stand for a simple microscope is shown in 

 fg. 10, where A is a brass pillar screwed to a tripod base ; B is a broad 

 stage for the object*, secured to the stem by screws, whose milled 



Fig. 10. 



heads are at c. By means of the large milled head D, a triangular bar, 

 having a rack, is elevated out of the stem A, carrying the lens-holder K, 

 which has a horizontal movement in one direction, by means of a 

 rack worked by the milled head F, and in the other direction by 

 turning on a circular pin. A concave mirror a reflects the light 

 upwards through the hole in the stage, and a lens may be attached to 

 the stage for the purpose of throwing light on an opaque object, in the 

 same way that the forceps H for holding such objects is attached. 

 uicroscope is peculiarly adapted, by iti broad stage and its genera] 

 itoodineot, fur dissecting; and it is rendered more convenient for this 

 purpose by placing it between two inclined planes of mahogany, which 

 support the arms and elevate the wrists to the level of the stago. 

 TMi apparatus ii called the dissecting rest. When dissecting is not a 



primary object, a joint may be made at the lower end of the stem A, 

 to allow the whole to take an inclined position ; and then the spring 

 clips shown upon the stage are useful to retain the object in its place. 

 Numerous convenient appendages may be made to accompany such 

 microscopes, which it will be impossible to mention in detail : the most 

 useful are Mr. Varley's capillary cages for containing animalculse in 

 water, and parts of aquatic plants ; also his tubes for obtaining and 

 separating such objects, and his phial and phial-holder for preserving 

 and exhibiting small living specimens of the Chara, Nitella, and other 

 similar plants, and observing their circulation. The phial-microscope 

 affords facilities for observing the operations of minute vegetable and 

 animal life. 



The mode of illuminating objects is one on which we must give some 

 further information, for the manner in which an object is lighted is 

 second in importance only to the excellence of the glass through which 

 it is seen. In investigating any new or unknown specimen, it should 

 be viewed in turns by every description of light, direct and oblique, as 

 a transparent object and as an opaque object, with strong and with 

 'aint light, with large angular pencils and with small angular pencils 

 thrown in all possible directions. Every change will probably develope 

 some new fact in reference to the structure of the object, which should 

 taelf be varied in the mode of mounting in every possibls way. It 

 should be seen both wet and dry, and immersed in fluids of various 

 qualities and densities, such, for instance, as water, alcohol, oil, and 

 Canada balsam, which laat has a refractive power nearly equal to that 

 of glass. If the object be delicate vegetable tissue, it will be in some 

 ispects rendered more visible by gentle heating or scorching by a 

 clear fire placed between two plates of glass. In this way the spiral 

 vessels of asparagus and other similar vegetables may be beautifully 

 displayed. Dyeing the objects in tincture of iodine will in some caaes 

 answer this purpose better. 



But the principal question in regard to illumination is the mag- 



Fig. H. 



nitude of the illuminating pencil, particularly in reference to trans- 

 parent objects. Generally speaking the illuminating pencil should be 

 as large as can be received by the lens, and no larger. Any light 

 beyond this produces indistinctness and glare. The superfluous light 

 from the mirror may be cut of by a screen having various-sized 

 apertures placed below the stage ; but the best mode of illumination is 

 that proposed by Dr. Wollaston, and called the Wollaston condenser. 

 A tube is placed below the stage of the instrument containing a lens A B 

 (jig. 11), which can be elevated or depressed within certain limits at 

 pleasure ; and at the lower end is a stop with a limited aperture c D. 

 A plane mirror E F receives the rays of light L L from the sky or a 

 white cloud, which last is the best source of light, and reflects them 

 upwards through the aperture in c D, so that they are refracted, and 

 form an image of the aperture at o, Which is supposed to be nearly the 

 place of the object. The object is sometimes best seen when the 

 image of the aperture is also best seen ; and sometimes it is best to 

 elevate the summit o of the cone A B a above the object, and at others 

 to depress it below : all which is dune at pleasure by the power of 

 moving the lens AB. If artificial light (as a lamp or candle) be 

 employed, the flame must be placed in the principal focus of a large 

 detached lens on a stand, so that the rays L L may fall in parallel lines 

 on the mirror, or as they would fall from the cloud. This will bo 

 found an advantage, nut only when the Wollaston condenser ii 

 employed, but also when the mirror and diaparagm are used. A gool 

 mode of irritating artificially the light of a white cloud opposite tho 

 sun has been proposed by Mr. Varley : he covera the surface of the 

 mirror under tho stage with carbonate of soda or any similar material, 



