86 



DR. L. SMITH'S INVERTED MICROSCOPE. 



box, pointing upwards towards the lower side of the stage; and it 



Fig. 43. 



Inverting Prism. 



is so attached that the coarse focal 

 adjustment may be made by sliding 

 it up and down, whilst the fine 

 adjustment is made by means of a 

 milled-head just above the prism- 

 box. The Illuminating apparatus is 

 of course placed above the stape, the 

 light having to be sent downwards 

 instead of upwards. Besides the 

 Mirror, there is an arm which may 

 carry Diaphragms, Polarizing prism, 

 &c. When it is desired to apply 

 Heat to an object, this is effected by 

 placing the glass whereon it lies upon 

 a plate of metal large enough to pro- 

 ject beyond the stage, and by apply- 

 ing to the projecting part of this 

 plate the flame of a spirit-lamp. 

 The Optical part of the instrument is 

 so fitted to the base, that it may be 

 entirely drawn away from beneath the stage, for the sake of chang- 

 ing the powers. Its action will be readily understood from an 

 inspection of the diagram (Fig. 43). The luminous rays which 

 pass downwards from the object through the objective, impinge 

 upon the prism at a perpendicularly to its surface ; when they 

 meet its first oblique surface at b they undergo total reflexion, by 

 means of which they are sent on to c, where they meet its second 

 oblique surface, and are again totally reflected, so as to pass forth 

 at d perpendicularly to its surface, and consequently without 

 refraction. — This instrument is extremely well adapted, not merely 

 for Chemical investigations, but also for the examination of any 

 objects (such as Diatomacea?) that sink to the bottom of the liquid 

 in which they are immersed ; since, by coming into contact with 

 the glass on which they lie, their surfaces are seen more exactly 

 in one plane than when viewed from above. It is also well 

 adapted for the purpose of Dissection ; the hands and instruments 

 being left much more free to work, when the object-glass does not 

 stand in their way.* 

 61. Nachet' s Double- Bo lied Microscope. — The division of the 



* The cost of this instrument, as made by MM. Nachet, and furnished 

 with four Objectives, Micrometer eye-piece, Goniometer, and other 

 accessories, is only 350 francs, or £14. — Dr. Leeson may fairly claim the 

 credit of an independent inventor as regards this form of instrument ; 

 one essentially the same having been constructed for him by Messrs. 

 Smith and Beck, at the same time that Dr. J. L. Smith's pattern was 

 being worked out by MM. Nachet. See Mr. Highley's account of his 

 Mineralogical Microscope, in "Quart. Journ. of Micros. Sci." Vol. iv. 

 p. 261. 



