CONSTRUCTION OF THE MICROSCOPE. 4:9 



tion, he caused the other half to be intercepted by a pair of prisms dis- 

 posed as in Fig. 27, 2, and to be by them transmitted to the other eye. 

 It will be readily understood that this arrangement, though pseudosco- 

 pic for the Compound Microscope, is stereoscopic for the Simple Micro- 

 scope, 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. Reek'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 narrowness of its field of view, 

 by its deficiency of light and of magnifying power, and by the inconve- 

 nience of the manner in which the eyes have to be applied to it. An ar- 

 rangement greatly superior in all these particulars having been since 

 worked out by MM. Nachet, the Author has combined this 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 aperture 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 27, 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 reflected 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 con- 

 vergence to the axes when the eyes are applied to the eye-tubes super- 

 posed 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 ( 38). The magnifying power of this instrument 

 may be augmented to 35 or 40 diameters, by inserting a conceive lens in 

 each eye-piece, which converts the combination into the likeness of a 

 G-alilean telescope (or opera-glass); and this arrangement (originally sug- 

 gested by Prof. Briicke of Vienna) has the additional advantage of 

 increasing the distance 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 delicate dissection, 

 the Author can most strongly recommend MM. Nachet's instrument. 

 No one who Jias 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 dissecting instrument to it in the vertical direction. 

 This is especially important when fine scissors are being used horizon- 

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

 through the plane desired, with an exactness totably unattainable by the 

 use of any monocular magnifier. 



48. Field's Dissecting and Mounting Microscope. This instrument, 

 constructed on the plan of Mr. W. P. Marshall, is a combination of a Dis- 

 secting Microscope with a set of apparatus and materials far the prepara- 

 tion and mounting of microscopic objects; the whole being packed in a small 

 cubical case about seven inches each way, convenient both for general use, 

 but more particularly as a travelling case for carrying the several requisites 

 for the examination and mounting of objects into the country, or to the 

 seaside. The Microscope can be used either Simple or Compound, as 

 shown in Fig. 37; and is fitted with a mirror, side-condenser, and stage- 

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