176 MANAGEMENT OF THE MICROSCOPE. 



provement of the higher powers of the Microscope, they have 

 thought comparatively little of the lower ; and in Continental 

 Microscopes, it is rare to meet with an objective which will give 

 even a tolerable view of a large opaque object. The Author, in- 

 deed, well remembers the time, when it was not thought worth 

 while, even by English Opticians, to construct Achromatic ob- 

 ject-glasses of less than an inch focus ; and the production of 

 objectives of IJ inch and 2 inch focus has been chiefly called 

 for, in consequence of their value in displaying anatomical pre- 

 parations in which the bloodvessels have been injected with 

 coloring matter. The view which is afforded of large opaque 

 objects, however, by a Compound Microscope, furnished with 

 even an imperfectly corrected Achromatic object-glass, giving a 

 magnifying power of 20 or 25 diameters, is so greatly preferable 

 to that which is given by any Simple Microscope, that no instru- 

 ment that is intended for general research should be unfurnished 

 with such a power. It is especially required in Microscopes that 

 are to be used for Educational purposes ; since it is most im- 

 portant that the young should be trained in a knowledge of the 

 wonders and beauties of the familiar objects around them ; and 

 an objective of low power and wide aperture, adapted to the 

 examination of a large surface at once, affords a means of dis- 

 playing these, such as can be afforded in no other way, save by 

 the use of the Erector and draw-tube (§ 44). A microscope fur- 

 nished with these appendages, need not be supplied with an ob- 

 jective of longer focus than 1 inch or 8-lOths inch ; but the 

 Author would strongly recommend to such as do not possess 

 them, that they should give to a " dividing" IJ inch or 2 inch 

 (in which the front lens is removable, and is replaced by a per- 

 forated cap that limits the aperture of the back lens, which is 

 then employed by itself, having a focus of about 3 inches) a pre- 

 ference over such as do not thus supply the extremely low power 

 which he recommends.' 



92. The mode of bringing opaque objects under view, will 

 differ according to their " mounting," and to the manner in 

 which it is desired to illuminate them. If the object be mounted 

 in a " slide" of glass or wood, upon a large opaque surface, the 

 slide must be laid on the stage in the usual manner, and the 

 object brought as nearly as possible into position by the eye 

 alone (§ 84). If it be not so mounted, it may be simply laid upon , 

 the glass stage-plate, resting against its ledge ; and the diaphragm- 

 plate must then be so turned, as to afford it a black background. 



' A single pair (flint and crown) of about 2 inch focus, was constructed at tlie 

 Autlior's request, some years since, by Messrs. Smith and Beck, for the special purpose 

 of exhibiting injected preparations, and other opaque objects; and its performance has 

 been so satisfactory to him, that he was induced to urge upon the Microscopic Committee 

 appointed by the Society of Arts, that the Educational Microscope for which they in- 

 vited competition (§ 31), should be furnished with such a power. This recommenda- 

 tion having been adopted, the instrument selected has been specially fitted for the 

 class of objects above alluded to. 



