40 VISION WITH THE COMPOUND MICROSCOPE 



corrected for a certain length of draw-tube, and, in order that they 

 may work efficiently, that definite length of draw-tube must be 

 maintained. 



In general it is not advisable to use with an achromatic objective 

 a greater super-amplification than can be obtained with a 10-power 

 eye-piece, or with an apochromatic objective that yielded with a 

 12 or 18 power one. 



We shall facilitate the comprehension by the student of the 

 principles of the modern form of a compound microscope by means 

 of fig. 30. In this figure the optical portion, that is, the objective and 

 eye-piece, are drawn to the full size, but the distance between these 

 has, from the exigencies of space, been much curtailed. A low- 

 power objective has been specially chosen for simplicity, and a com- 

 pensating eye-piece (vide Chapter Y.) has been introduced to show 

 its form and mode of action. 



The objective is a copy of an old Ross 1-inch of 1856. The 

 incident front (that is, the lens on which the incident beams from 

 the object first strike) is a convex of long radius ; the incident sur- 

 face of the flint lens of the back combination is a concave of very 

 long radius, being in fact about twenty inches. 



The object F has only rays drawn from one side in order that 

 a clearer perception of the path of the rays may be seen. This pair 

 of rays passes from the arrow (object) through the combination of 

 lenses forming the objective, giving an inverted real image at A B. 

 This image, in fact, has a convex curve towards the eye-piece : this 

 is a position that will tend to increase the curvature of the virtual 

 image C D given by the eye-piece, the inverted image (A B) at the 

 diaphragm of the eye-piece being the subject of still further and 

 often great magnification. 



In addition to the two lenses of which the compound microscope 

 may be considered to essentially consist, it was soon found needful 

 to introduce another lens, or a combination of lenses, between the 

 object-glass and the image formed by it, the purpose of this being 

 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-glass. As it thus allows more of 

 the object to be seen at once, it has been called the field-glass ; but 

 it is now usually considered as belonging to the ocular end of the 

 instrument, the eye-glass and the field-glass being together termed 

 the eye-piece, or ocular. Various forms of this eye-piece have been 

 proposed by different opticians, and one or another will be preferred 

 according to the purpose for which it maybe required. That which, 

 until the construction of the compensation eye-pieces by Abbe, was 

 considered the most advantageous to employ with achromatic object- 

 glasses, to the performance of which it is desired to give the greatest 

 possible effect, was termed the Huyghenian, having been employed 

 by Huyghens for his telescopes, although without the knowledge of 

 all the advantages which its best construction renders it capable of 

 affording. This eye-piece, with others, will be considered in detail 

 in the chapter (v.) given in part to their consideration ; but this 

 eye-piece consists of two plano-convex lenses, with their plane sides 



