MECHANICAL ARRANGEMENTS. 47 



the illuminating pencil is reduced to about 20°. The same point may 

 be attained, although with much sacrifice of definition, by gradually 

 depressing the condenser, so that the rays may diverge before they 

 reach the object ; and it may be remarked, generally, that the defi- 

 nition of objects is always most perfect when an illuminating pencil of 

 suitable form is accurately adjusted to focus, that is, so that the source 

 of light and the plane of vision may be conjugate foci of the illumi- 

 nator. If an object-glass of 120° aperture, or upwards, be used as an 

 illuminator, the markings of Diatomacese will be scarcely distinguish- 

 able with an object-glass, the glare of the central rays overpowering 

 the structure of those that are more oblique." 



Mechanical Aerangements. 



Having now explained the more important optical principles of the 

 achromatic compound microscope, it remains only to describe the me- 

 chanical and accessory arrangements for giving those principles their 

 full effect. The mechanism of a microscope is of much more import- 

 ance than might be imagined by those who have not studied the sub- 

 ject. In the first place, steadiness, or freedom from vibrations which 

 are not equally communicated to the object under examination and to 

 the lenses by which it is viewed, is a point of the utmost consequence. 



One of the best modes of mounting a compound microscope is 

 that shown, fig. 35, which, although it does not exhibit all the details, 

 will serve to explain the chief features of the arrangement. 



" The mechanical construction," says Mr. Ross, " is derived from 

 a practical acquaintance with the various improvements made in the 

 microscope for many years. The general arrangement, which is pro- 

 perly the province of the mechanic, has been contrived to obtain the 

 utmost freedom from tremor, and to afford the greatest facility in 

 using the various movements; while the extent, direction, and num- 

 ber of these have been collected from the experience of the most 

 indefatigable observers in all the various branches of microscopic 

 inquiry. 



The optical part, also, has arrived at such perfection, that points 

 or lines whose distance is such that their separation is bordering on 

 interfering with the physical constitution of light can be distinctly 

 separated, thus insuring a reality in the appearance of objects where 

 the minuteness of the detail approaches the natural Hmit of micro- 

 scopic vision." 



In the larger instrument, fig. 36, two uprights, J, are strengthened by 



