In addition to the two lenses of which the compound microscope has been stated essen tially to consist, another is usually introduced between the object-glass and the image formed by it. The ordinary purpose of this lens is to change the course of the rays in such a manner that the image may be formed of dimensions not too great for the whole of it to come within the range of the eye-piece, and consequently to allow more of the object to be seen at once.
Hence it is called the field-glass (fig. 163). It may be so adjusted, however, in regard to the eye-glass, as to correct its errors in almost a perfect degree; and it is now, therefore, usually considered as belonging to the ocu lar end of the instrument,—the eye-glass and the field-lens being together termed the eye piece. Various forms of this eye-piece have been proposed by different opticians, and one or other will be preferred, according to the purpose for which it be required. It may be laid down as a general principle, however, that to give the highest effect to the microscope, in regard to clearness of view and penetrating power, no more than two lenses should be employed; and that when a certain amount of these may be sacrificed to gain a large flat field, three is the largest number which can be introduced with any benefit. This prin ciple is founded on the fact that, whenever light impinges on the surface of even the most transparent body, a part of it only is trans mitted, the remainder being reflected. In the passage of light through ordinary lenses, there fore, a certain quantity is lost by reflection at each surface; and every multiplication in the number of lenses entails, therefore, a positive evil, which may or may not be counterbalanced by the good it effects. In the doublet or triplet already described, the correction of the aberrations is an advan tage much greater than the injury resulting from the substitution of four or six surfaces for two ; but this is by no means the case in the eye-piece, in which (from their low power) the aberrations are much less. Hence, when too many lenses are employed in it, although the field of view (that is, the circle within which the image is comprehended) may be very much enlarged and rendered flatter, the brilliancy and sharpness of the image are so much im paired, and it is invested with so much false colour, that, for all scientific purposes, the instrument is rather deteriorated than im proved.
The eye-piece which may be most advan tageously employed with achromatic object glasses, to the performance of which it is desired to give the greatest possible effect in regard to defining and penetrating power, with out the necessity of a large field, is that termed the Huyghenian, having been employed by Huyghens for his telescopes, although without the knowledge of all the advantages which its best construction rendered it capable of afford ing. It consists of two plano-convex lenses with their plane sides towards the eye. These are placed at a distance equal to half the sum of their focal lengths ; or, to speak with more precision, at half the sum of the focal length of the eye-glass, and of the distance from the field-glass at which an image of the object glass would be formed by it. A stop or dia phragm must be placed half-way between the two lenses. By Huyghens this arrangement was intended merely to diminish the spherical aberration ; but it was subsequently shown by 13oscovich that the chromatic dispersion was also in great part corrected by it. Since the introduction of achromatic object-glasses for compound microscopes, it has been further shown that all error may be avoided by a slight over-correction of these, so that the blue and red rays may be caused to enter the eye in a parallel direction, and thus to produce a colour less image, though not actually coincident (fig.
164). Further, the image produced by the meeting of the rays after passing through the field-glass is by it rendered convex towards the eye-glass, instead of concave, so that every part of it may be in focus at the same time, and the field of view thereby rendered flat. Those who desire to gain more information upon this sub ject than they can from the accompanying dia gram and the explanation of it, may be re ferred to Mr. Varley's investigation of the pro perties of the Huyglienian eye-piece in the 51st volume of the Transactions of the Society of Arts, and to the article "Microscope" in the Penny Cyclopwdia.