ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 97 



up to the first lens of the eye-piece, their rays being parallel. To 

 this parallelism it is due likewise that every disturbing effect (like 

 that which the thickness of the cover-glass exerts) by the prisms ou 

 the transmitted pencil is excluded, for such effects can only be pro- 

 duced by converging or diverging pencils. 



The mode of using an instrument so constructed does not differ 

 from that of an ordinary Microscope, except that first the two eye- 

 pieces must be removed and adjusted for infinite distance, and then 

 replaced. By means of the adjusting movement the left eye-piece 

 tube is then put in such a position that with proper illumination the 

 two diaphragm apertures of equal size, which are inside the eye- 

 pieces, are seen without effort as one ; an object being now introduced 

 and brought into focus, the plastic image infallibly appears, and 

 cannot be seen double. To produce this effect in perfection, however, 

 the position of the prisms must be so adjusted that the images 

 together with the diaphragm apertures become merged into one com- 

 plete whole, and the impression is produced of looking through a 

 round opening at the object which is behind. After this position of 

 the prisms has been once fixed no focussing that may be necessary 

 alters the effect. The figure shows that the half of the rays which 

 pass to the second prism is that furthest from the observer ; in the 

 opposite case the effect would be pseudoscopic. 



Plane mirrors of glass may be used instead of the prisms, but 

 the surfaces of both the prisms and the mirrors must of course be 

 perfect. The prism which is inserted half-way, A, is best made 

 equilateral, because with a rectangular one the total reflection might be 

 questionable, and the edge is better ; the other may be rectangular, 

 and should be of such a size that when the first is re- 

 moved it can take in and reflect the full pencil of rays ; ^^^- ^• 

 we then have a monocular Microscope. It is obvious that fll 

 instead of the eye-pieces described, actual achromatic j_i 

 telescopes could be used." ^m 



Hartnack's Demonstration Microscope.* — This Jt'm, 



(Fig. 5) consists of a tube, carrying eye-piece and objective, ir-lffl 



fixed to a frame by which it can be held in the hand. A |||_||i 



micrometer screw a serves for focussing the object which i'^li' 



is fixed to the circular stage by clamps. The continua- 1^1 



tion of the stage forms a metallic drum, at the lower end ^^^W 



of which is a convex lens L to concentrate light on the r: 1 1 



object. A diaphragm-disk is inserted in the drum with ,: | I 



a portion of its margin projecting on one side so as to be ^^^^ 

 revolved by the finger. ^ 



Lacaze-Duthiers' Microscope witli Eotatinsf Foot. — M. Nachet 

 has supplied us with a drawing (Fig. 6) of a Microscope similar to 

 that which we described at p. 873 of Vol. III. It is the device of 

 Professor H. de Lacaze-Duthiers. 



The speciality of the instrument is that the bottom of the pillar 



* Thanhoffer's ' Das Mikroskop und seine Anwendung,' 1 SSO p 55 

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Ser. 2.— Vol. II. H 



