nachet's binocular magnifier. 55 



understood that this arrangement, though pseudoscopic for the 

 Compound Microscope, is Stereoscopic for the Simple Microscope, 

 in which there is no reversal of the pictures ; and the Author can 

 testify to the fidelity of the effect of relief obtainable by Mr. R. 

 Beck's apparatus, which, being carried on an arm superposed upon 

 that which bears the magnifier, can be turned aside at pleasure. 

 But he has found its utility to be practically limited by the narrow- 

 ness of its field of view, by its deficiency of light and of magnify- 

 ing power, and by the inconvenience of the manner in which the 

 eyes have to be applied to it. — An arrangement greatly superior in 

 all these particulars having been recently worked-out by MM. 

 Nachet, the Author has combined the Optical part of their Dissect- 

 ing Microscope with Mr. R. Beck's Stand, and finds every reason to 

 be satisfied with the result ; the solidity of the stand giving great 

 firmness, whilst the size of the Stage-plate affords ample room for 

 the hands to rest upon it. The Objective in Nachet's arrangement 

 is an Achromatic combination of three pairs, having a clear aper- 

 ture of nearly 3-4ths of an inch, and a power about equal to that 

 of a single lens of one-inch focus ; and immediately over this is a 

 pair of Prisms, each resembling a, Fig. 22, having their inclined 

 surfaces opposed to each other, so as to divide the pencil of rays 

 passing upwards from the Objective into two halves. These are re- 

 flected horizontally, the one to the right and the other to the left ; 

 each to be received by a lateral Prism corresponding to B, and to 

 be reflected upwards to its own Eye, at such a slight divergence 

 from the perpendicular as to give a natural convergence to the 

 axes when the eyes are applied to the Eye-tubes superposed on 

 the lateral prisms, — the distance between these and the central 

 prisms being made capable of variation, as in the Compound 

 Binocular of the same makers (§ 29). The magnifying power of 

 this instrument may be augmented to 35 or 40 diameters, by 

 inserting a concave lens into each Eye-piece, which converts the 

 combination into the likeness of a Galilean Telescope (or Opera- 

 glass) ; and this arrangement (originally suggested by Prof. Brticke 

 of Vienna) has the additional advantage of increasing the dis- 

 tance between the object and the object-glass, so as to give 

 more room for the use of dissecting instruments. — To all who 

 are engaged in investigations requiring very minute and deli- 

 cate dissection, the Author can most strongly recommend MM. 

 Nachet's instrument. No one who has not had experience of it 

 can estimate the immense advantage given by the Stereoscopic 

 view, not merely in appreciating the solid form of the object under 

 dissection, but also in precisely estimating the relation of the in- 

 strument to it in the vertical direction. This is especially impor- 

 tant when horizontal sections are being made with fine Scissors ; 

 since the course of the section can thus be so regulated as to pass 

 through the plane desired, with an exactness totally unattainable by 

 the use of any Monocular Magnifier. 



