862 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Hitherto, projection Microscopes have given a relatively large 

 magnifying power, but with a definition insufficient for the wants of 

 science. This arises from the quality of the objectives which are 

 employed, and also from the way in which the illumination is obtained. 

 We have recognized that according to the dimensions of the micro- 

 scopic objects to be projected and the magnification desired, it is 

 necessary to vary the form of the convergent pencil which illuminates 

 the object, consequently the focus of the additional lens must be 

 modified. The apparatus is therefore provided with lenses of different 

 foci to be used with the condenser, according to circumstances. 



We have, morever, arranged to employ the objectives used for 

 ordinary Microscopes. Thanks to these and to the perfection of our 

 condensing system we are able to project microscopic objects with 

 high powers and with a clearness as perfect as that obtained with the 

 ordinary Microscope." 



Polarizing prisms can be used, also a rotating stage, so that 

 sections of rocks and crystals can be projected. 



"Twin" Simple Microscope. — Fig. 187 shows a peculiar arrange- 

 ment of two simple Microscopes mounted side by side on one plate. 

 One of them is fitted with a power of about 1 in., and 

 Fig. 187. the other about 1/4 inch, and both have Lieberkiihns. 

 jr^^ ^"^j» The object is held by forceps pivoted beneath the lens- 

 Ww '^^ if carrier, so that it can be readily examined by either 

 -* ^'^ ' power without having to alter the lenses, as is ordinarily 

 the case. 



The instrument must have been made a considerable 

 time, for we have seen an exactly similar one in the 

 " Cabinet de Physique " of the University of Louvain, 

 where we vp^ere informed it had been for upwards of 30 

 years. The workmanship suggests a French origin. 



Laurent's Apparatus for registering the Curva- 

 ture and Refraction of Lenses.* — M. L. Laurent's 

 apparatus consists of a vertical frame B (fig. 188), in 

 which slides a rectangular carrier S controlled by a 

 chain ; the position of the carrier is shown by a vernier, 

 and the lenses to be tested are placed on a plate 

 rotating horizontally on the carrier. On the top of 

 the frame is an eye-piece having a diaphragm (shown 

 in front view at D, fig. 189) divided in two parts : the 

 right half is covered by an illuminating prism; its 

 horizontal face is silvered, and squares are ruled on the 

 silver, which are viewed either by refraction or re- 

 flection ; the image of the squares is seen in the plane of the 

 diaphragm. 



A plane plate of glass T is put on the carrier S, and the vernier is 

 adjusted to zero at the point p where the plane is seen to touch the 

 squares. The lens L is placed on the plane; the light emerging 



* Comptea Kendus, c. (1885) pp. 903-5 (4 figs.). 



