THE MICROSCOPE. 35 



more convenient stage and diaphragm are added, Hart- 

 nack's microscope will still be preferred. What is wanted 

 is a stage of blackened glass. The clips upon Mr. Swift's 

 microscope are faulty, and should be improved, and the 

 condenser would be more convenient if fixed to the body 

 of the instrument, as in Hartnack's microscope, and not 

 placed on a separate stand, which always gets in one's way 

 when working. The microscope, however, costs less than 

 Hartnack's, and surely it will not be difficult to remedy its 

 slight defects. When that is done, it will be a better micro- 

 scope than Hartnack's. A nose-piece, and in that case a 

 rack-and-pinion coarse adjustment, would render it a very 

 perfect microscope for all ordinary work. 



If a more expensive microscope stand be desirable, one 

 similar to Hartnack's No. VII. stand should be procured. 

 It is perfectly steady. The stage is covered with blackened 

 glass. It has an easily worked rack-and-pinion coarse 

 adjustment. The stage is not too high, and it together 

 with the tube and lenses can be turned round a vertical 

 axis, for the variation of the illumination of the object. 

 Any English optician could make such a stand ; meanwhile 

 Hartnack's may be safely recommended. 



49. Binocular Microscope If the rays transmitted from an 

 object through each lateral half of the objective be focalised on the corre- 

 sponding retina, so that two slightly dissimilar pictures are produced, 

 the effect is stereoscopic ; the object stands out en has relief. Various 

 methods have been invented for effecting this, most of which imply 

 a permanent division of the tube of the microscope into two. Hart- 

 nack's arrangement is the best, for it is entirely confined to a special 

 double-tubed eye-piece, which may at any time be placed in the 

 ordinary monocular microscope. In this eye-piece there are four rec- 

 tangular prisms (Fig. 33), in which the rays undergo total reflection. 

 In two of these (A B) the rays from the objective are divided into two 

 lateral bundles, that are again reflected by the prisms C and D, and 

 focalised by the ordinary lenses of two eye-pieces at E and F. Such 

 an arrangement may be satisfactorily employed with low powers, but 

 with very high powers the loss of light entailed by the rays encounter- 

 ing so many glass surfaces, even at right angles, is a serious drawback, 

 and definition is apt to be impaired. Therefore, although it is less 

 fatiguing for the eyes to use the binocular, the monocular is still the 

 microscope for ordinary work. 



