24 CONSTRUCTION OF 



The lenses are mounted in cells as shown at F, and are screwed 

 into the dovetail bar H, sliding between three stont pins III, the 

 nearest one having a strong pin on the under side which keeps the bar 

 in close contact with the other two without any shake. The bar is 

 moved across the object by either of the nuts N N, which instead of 

 having a pinion, as usual, have a spring wound round their axis 

 attached at each end to the bar H, with an adjusting screw to regulate 

 the tension at the end J ; the bar may also be turned round on the 

 central pin fitted in the top of the stem A, and thus a traversing 

 motion in every direction may be given to the bar and lens, without 

 disturbing the object or altering in the least the distance between it 

 and the lens. The adjustment of the focus is first made by sliding 

 the stage pieces O P by the hand until the object is seen nearly distinct, 

 the thumb screw R being then turned, fixes the lower piece P to the 

 stem A ; then by means of the large milled head S, the final adjustment 

 is made by the intervention of a connecting bar T attached to the 

 stage piece ; this bar works on an elastic eccentric movement under 

 the milled head S, so that an adjustment of any small quantity can be 

 obtained with extreme precision. The slider containing the object 

 is kept close to the stage by two heliacal springs V V. A condensing 

 lens and a pair of forceps are made to fit in the piece O, and can be 

 employed with or without the stage-plate, which may be entirely 

 removed by the thumb-screw in front when necessary. 



We have been thus particular in the description of single 

 microscopes, although their use in the present day is extremely limited, 

 for two reasons, first, because a perfect comprehension of that instru- 

 ment better enables us to understand the principles of the more 

 perfect apparatus, the compound microscope ; and, secondly, because 

 such instruments are still in use, and this Manual would therefore 

 have been incomplete without such a description. For certain kinds 

 of botanical specimens, the single microscopes are exceedingly useful. 

 Figures 9 and 10 represent two forms of botanical microscopes, which 

 however require no very extended explnnation. The latter instru- 

 ment, fig. 10, is indeed a very superior apparatus, both from its por- 

 tability and exceedingly moderate price. It is fitted with good object 

 and eye-glasses, as well as with a mirror, which can be turned to any 

 required angle. It may also be obtained, fitted with additional powers 



