MICROSCOPE. 



535 



dividing prism or prisms. The reason of this 

 will appear from a simple experiment, which 

 any oue can easily make ; and in judging of the 

 performance of a binocular, allowance for this 

 must be made. It will be at once perceived 

 that the images presented to either eye come 

 from separate halves of the objective. If we 

 take a finely-corrected objective, and carefully 

 adjust it on some test, and then, by means of a 

 strip of paper, cover half the opening, at the 

 back, thus cutting off half the pencil, and pre- 

 senting exactly the condition of the objective 

 as used to form the binocular images, it will be 

 found to have lost considerably in fine defini- 

 tion, but to have gained in penetration, 

 using this latter term to signify the power 

 of perceiving a? in focus, or nearly so, at the 

 same time, different depths of the object. 

 Now, when the binocular vision is effected, 

 by whatever means, there will be this slight 

 loss of definition almost inappreciable in pow- 

 ers below the ,* ff th inch, but in some de- 

 gree compensated by a great gain in penetra- 

 tion, which is of greatest value in assisting in 

 the stereoscopic perception of the objects; and 

 nothing can be more truly wonderful and pleas- 

 ing than the effect of Mr. Tolles' eye-piece, 

 especially with the lower powers, and upon 

 opaque objects. Anatomical objectives, both 

 transparent and opaque, are exhibited with a 

 beauty entirely lost when viewing them in 

 the ordinary manner. The Ith inch objective 

 works remarkably well with the binocular eye- 

 piece, as also the j-^th inch, the fields illumi- 

 nating readily with the achromatic condens- 

 er ; there is, however, as already stated, 

 some loss in definition, though at times hardly 

 appreciable with opaque objects this loss of 

 definition is much less perceptible, and the 

 stereoscopic effect is very line. The principle 

 upon which this eye-piece is constructed is 

 simple; the optician will readily understand 

 it. when told that the division of the pencil is 

 effected at the crossing point of an achromatic 

 erecting eye-piece. The electing eye-piece of 

 Mr. Tolles consists essentially of two Huyghe- 

 nian eye-pieces, and is much superior to the 

 old form; the anterior combination, or that 

 nearest the objective, is almost half the power 

 of the ordinary A., and about double its 

 length. The field lens is a double achromatic, 

 and the eye lens a triple achromatic they are 

 placed much farther apart than in the ordinary 

 negative eye-piece. Immediately above the 

 triple achromatic is placed the dividing prism ; 

 the arrangement is that of Xachet, thus giving 

 orthoscopic vision, inasmuch as the image may 

 be considered as a real object. It will be at 

 once perceived, that the bad effect sometimes 

 produced in Mr. Wenhanrs arrangement, aris- 

 ing from the necessity of placing the dividing 

 prism so far from the posterior lens of the ob- 

 jective, is here entirely obviated, the division 

 being effected exactly at the point required 

 there is a sliding motion allowed to the lenses 

 anterior to the dividing prism which permits a 



perfect compensation when different objectives 

 are employed ; it is however, hardly necessary. 

 The eye-piece projects beyond the ordinary 

 tube by the length of the prisms and eye- 

 pieces ; the latter are of the usual form of nega- 

 tive eye-pieces, and may be varied to increase 

 the power ; they are applied almost hi contact 

 with the prisms. The two tubes are parallel, 

 and adjusted by a screw to open or close sym- 

 metrically, to suit the width of eyes of different 

 observers. As yet, Mr. Tolles has sent out but 

 the one eye-piece, now before the writer, but 

 will soon be ready to supply the demand which 

 will certainly be made for them. 



The next improvement we shall notice is an 

 entirely new method of illuminating opaque 

 objects, to be viewed by the higher powers ith 

 inch to ^gth inch, an invention of the writer, 

 and here first described. The difficulty in 

 illuminating opaque objects, to be viewed with 

 the higher powers, is so great, that it is rarely 

 attempted ; an ingenious arrangement of lieb- 

 erkuhns has been made by Ross, as suggested 

 by Mr. Brooke, but it is of difficult and limited 

 use. Of course, if it be attempted to illuminate 

 by means of condensers applied at the side, as 

 suggested by Mr. Richard Beck, the light 

 thus thrown in must cast immensely long 

 shadows, and could hardly be sent in at all, 

 under a y^th inch or a y^th inch. Mr. Wen- 

 ham has proposed to effect the illumination 

 of balsam mounted objects, by means of 

 reflection from the glass cover, the light be- 

 ing caused to impinge from below, at such 

 angle as to be totally reflected. This meth- 

 od is only applicable to mounted objects, 

 and has not proved as advantageous as was 

 expected, especially in viewing the diatomace- 

 ous frustule, which becomes too transparent 

 when mounted in balsam, to throw back much 

 light, when thus illuminated. It was to meet 

 this latter want that the writer devised the 

 simple plan now to be described. The employ- 

 ment of a collimating eye-piece, with a transit 

 instrument, suggested it. The idea is briefly 

 this: to make the objective itself the illumi- 

 nator. To effect this, a small bit of tube open 

 at one side, carrying a movable perforated 

 silver reflector, is screwed directly behind the 

 objective, and can be turned in any direction 

 toward the light. An ordinary small illuminat- 

 ing lamp gives most intense illumination, 

 with the Tjth inch, without any condensing 

 lens outside. There are some special con- 

 trivances necessary, to prevent reflection of 

 light up to the eye-piece, from the surface of 

 the posterior lens of the objective, thus obscur- 

 ing the vision. This is easily effected, and clear 

 brilliant illumination with a dark field readily 

 obtained. We need not here describe it more 

 particularly, as it will soon be put into the 

 hands of Messrs. Wales & Co., with special care 

 to its proper construction, and it will be 

 described more fully, and the necessary mani- 

 pulations, in another place. The revelations 

 had by means of this Uluminatoi' are wonder- 



