CONSTRUCTION OF THE MICROSCOPE. 57 



to allow the rays reflected from the concave side to be either brought to a 

 focus on the object, or to give a uniform illumination over a larger field, 

 and this alike with the parallel rays of daylight, and the diverging rays 

 of a lamp; and (2) that it may be thrown so far out of the optic axis, as 

 to reflect rays of considerable obliquity. The first of these objects is 

 answered by making the mirror-frame slide upon a stem fixed into the 

 bottom of the pillar (Fig. 41); but this does not give sufficient obliquity. 

 The second is readily provided-for by attaching the mirror-frame to a 

 swinging-bar, pivoted to the under side of the stage (Fig. 42); this gives 

 any amount of obliquity, but does not enable the distance of the mirror 

 from the stage to be varied. If the mirror-frame be made fco slide on a 

 stem, it should be mounted on a jointed arm, so as to be made capable of 

 reflecting very oblique light; or, if attached to a swinging bar, this bar 

 should be made capable of elongation by a sliding piece working in a 

 dove-tail groove (as in Wale's Microscope, Fig. 44), so as to allow its dis- 

 tance from the stage to be varied. A very ingenious arrangement of the 

 rotating 'upper stage' has been devised by Mr. John Phin (of New 

 York-). It is so fitted with a short tube, that it may be slid into the 

 cylindrical fitting, not only from above, but also from below; and as the 

 object-slide rests upon the springs which press it upwards against the 

 stage-plate, not only may light of any degree of obliquity be thrown upon 

 it, but the advantage of a ' safety-stage ' ( 117) is obtained, since the 

 springs that support the slide readily yield to any pressure exerted on it 

 by the objective. A Student's Microscope fitted with this form of rotat- 

 ing stage, and with either Wenham's ( disk illuminator,' or ' Woodward's 

 prism' ( 101), and having the mirror hung in the manner just recom- 

 mended, will be found capable if furnished with good Objectives of 

 resolving all but the most difficult Diatom-tests. 



55. In regard to the qualities of the Objectives desirable for a Stu- 

 dent's Microscope, the Author feels assured that he expresses the convic- 

 tion of the most experienced ivorkers in various departments of Biological 

 inquiry, when he re-affirms the doctrine of which nearly half a century's 

 varied experience has satisfied him, but which has been of late vehe- 

 mently contested (not always very cautiously) by Microscopists whose 

 range of study has been less extended that good definition, with mode- 

 rate angle of aperture, is the essential requisite; Objectives of this class 

 being not only much more easy to use by the inexperienced, but fre- 

 quently also giving much more information even to the experienced (in 

 virtue of their greater ' penetration ' or 'focal depth'), than can be ob- 

 tained from Objectives of the very wide angles required for the resolution 

 of difficult diatom-tests (see 161). Every one who is at all conversant 

 with the recent history of Mi^cro-Zoology, Micro-Botany, Micro-Geology, 

 or Animal or Vegetable Histology, must know that at least ninety-nine 

 hundredths of the enormous additions made to each of these departments 

 of inquiry during the last quarter of a century, have been worked-ont by- 

 Objectives of the kind here recommended; and those who affirm that all 

 this work is so imperfect that it will have to be done over again with Ob- 

 jectives of excessively wide aperture, have to prove the fact. Doubtless 

 new methods of preparation are constantly revealing novelties in whole 

 classes of objects which (it was supposed) had been already studied 

 exhaustively; and no one can affirm that he has made out everything, in 

 any object, which it is capable of being thus made to show. But the 

 Author feels confident that no such extension of our knoAvledge is likely 

 to take place in this direction, as will require the habitual use of the very 



