88 THE MICROSCOPE AND ITS REVELATIONS. 



ted to it, is completely pushed-in, the acting length of the body (so to 

 speak) is so greatly reduced by the formation of the first image much nearer 

 the objective, that, if a lens of 2-3ds of an inch focus be employed, an ob- 

 ject of the diameter of l inch can be taken in, and enlarged to no more 

 than 4 diameters; whilst, on the other hand, when the tube is drawn-out 4^ 

 inches, the object is enlarged 100 diameters. Of course every interme- 

 diate range can be obtained by drawing-out the tube more or less; and the 

 facility with which this can be accomplished, especially when the Draw- 

 tube is furnished with a rack-and-pinion movement (as in Messrs. Beck's 

 Compound Dissecting Microscope), renders such an instrument very use- 

 ful in various kinds of research. 



85. Micro-Megascope. This designation has been applied by Dr. J. 

 Matthews, 1 to an arrangement of the ordinary Microscope, whereby such 

 a low amplification may be obtained, as gives a general view of large ob- 

 jects, without the need of any special apparatus. The method consists 

 in employing the ordinary microscope to magnify not the object itself 

 but an image of it formed by a lens placed between the object and the 

 front of the objective. In the principle of this method there is nothing 

 new, for every Microscopist who has focussed an Achromatic Condenser, 

 upon a transparent object, has seen the image formed by it of his win- 

 dow-frame, blind-tassel, or (it may be) of sharply defined clouds. 3 And 

 Dr. Royston-Pigott has been accustomed to employ such images of hairs, 

 fine wires, etc., as 'tests' for the defining quality of Objectives of high 

 magnifying power. The novelty consists in the mode of applying it to 

 the purpose just named. This answers best when an Objective of 

 2-inches or IJ-inch focus is used in the microscope, and a 1-inch Objec- 

 tive is placed in the Sub-stage with its front-lens upward. The object to 

 be imaged by the latter is to be placed either at some distance behind 

 it, the mirror being turned aside, or, if the Mirror be employed, at some 

 distance from it on either side; the distance, in either case, being adapted 

 to give to the Microscopic image the amplification required. The former 

 arrangement is most convenient if the Microscope is being used in a hori- 

 zontal position; the latter is most suitable when the Microscope is inclined, 

 the distance of an object placed in the optic axis being then limited. If 

 exact definition is required, the Mirror should be replaced by a right- 

 angled Prism ( 2). The object, whether transparent or opaque, must 

 be suitably illuminated; and it will be found convenient to use a special 

 support so made that its position and height may be conveniently varied. 



86. NaclieVs Erecting Prism. An extremely ingenious arrangement 

 has been made by MM. Nachet, on the basis of an idea first carried into 

 practice by Prof. Amici, by which the inverted image given by the Com- 

 pound Microscope is erected by a single rectangular prism placed over the 

 eye-piece. The mode in which this prism is fitted up is shown in Fig. 

 60 (2); the rationale of its action is explained by the diagram (1). The 

 prism is interposed between the two lenses of the Eye-piece, and has 

 somewhat the form of a double wedge, with two pentagonal sides, A B c D 

 E, and A B H G F, which meet each other along the common edge A B, 

 and two facets, D E F G, and c D G H, which meet along the common 

 edge D G, the edges A B and D G being perpendicular to each other. The 

 rays emerging from the field-glass enter this prism by its lower surface, 

 and are reflected at I, upon the face A B H G F, from which they are again 



1 " Journal of Quekett Microscopical Club," July, 1879. 



2 The Author has thus exhibited to his friends a Microscopic view of the Moon, 



