14 HOW TO WORK 



examining living objects in vivaria. It is fixed to the glass of the 

 vivarium by means of a pneumatic arrangement. It costs three 

 guineas. This is made by Mr. Collins, of Titchfield Street. 



2O. Clinical, Pocket, Travelling?, and Class Microscope. Under 

 this head I propose to describe an instrument devised by me some 

 years since which I have found very useful for ordinary observation, 

 in the field, and also for medical work, and it has been employed 

 with great success for class demonstration. 



The Microscope. Like some other instruments which have from 

 time to time been proposed, this microscope is composed of draw- 

 tubes like a telescope ; but the arrangement of the stage, and the 

 plan adopted for moving the slide when different parts of the 

 object are submitted to examination, differ entirely, as far as I am 

 aware, from those hitherto proposed. The instrument consists of 

 three tubes, a, l>, c, fig. 21, plate VII ; a carries the eye-piece, is four 

 and a-half inches long, and slides in b, which is of the same length, 

 but only slides up to its centre in the outer tube c. Tube b carries 

 the object-glass. The tube c can be fixed by aid of a screw ring d, 

 at any height, according to the focal length of the object-glass. 

 This arrangement prevents the object-glass from being forced through 

 the preparation by careless focussing. At the lower part of the body 

 is an aperture for throwing the light on opaque objects. The pre- 

 paration is kept in contact with the flat surface below by a spring, 

 which allows the requisite movements to be made with the hand, 



figS. 22, 23. 



That part of the object which it is desired to examine can be 

 easily placed opposite the object-glass if the instrument is inverted. 

 The proper focus is obtained by a screwing movement of the tube 

 b\ and if it be desired to examine any other parts of the object, 

 this is easily effected by moving the slide with one hand, while the 

 instrument is firmly grasped with the other. Delicate focussing is 

 effected by drawing the tube a up and down. By this movement the 

 distance between the eye-piece and object-glass is altered. 



Any object-glass may be used with this instrument. I have 

 adapted various powers, from a three-inch, magnifying fifteen diameters, 

 to a twelfth, magnifying seven hundred diameters, and I feel sure that 

 even higher powers may be used. 



In the examination of transparent objects, p. 18, ordinary daylight 

 or the direct light of a lamp may be used ; or, if more convenient, the 

 light may be reflected from a sheet of white paper, or from a small 

 mirror inclined at the proper angle, and placed on the table. In 

 examining objects by reflected light, p. 18, sufficient illumination is 

 obtained from an ordinary wax candle placed at a short distance from 



