356 MANIPULATION. 



eye-piece, n, at its upper and an erector at its lower end, 

 is also moved up and down by rack and pinion, by turning 

 the milled head, L'. By the employment of an erector at 

 the lower end of the tube, K, this microscope becomes, in 

 every respect, similar to the compound instrument described 

 at page 69, and objects are not seen by it in an inverted 

 position, therefore it can be employed in dissecting. When 

 the tube, K, is turned down closely upon I, the object-glass, 

 o, is farthest from the object, and a magnifying power of eight 

 diameters is obtained ; but if this tube be turned out as far as 

 it will go, the object-glass must then be brought nearer the 

 object, and the magnifying power wiU be as much as one 

 hundred and thirty-five diameters. This microscope will be 

 found very convenient for many purposes where a great 

 amount of defining power is not required ; and as any vari- 

 ation in its magnifying between eight and one hundred and 

 thirty-five diameters can be readily obtained by turning the 

 milled heads, L and L', without the trouble of shifting the 

 object-glass, this point alone is sufficient to entitle it to a fair 

 share of praise. The author's attention was first directed to 

 this microscope by Dr. John Hughes Bennett, but the descrip- 

 tion was taken from a similar instrument in the possession of 

 Mr. C. H. Hallett. 



Another very excellent form of dissecting microscope is 

 that made by M. Nachet, and represented in fig. 236, 1 ; it is 

 mounted on a tripod stand, and, like the instrument of Powell, 

 before described in page 52, can be brought over the vessel in 

 which the dissection is being carried on. If, however, it be 

 required to examine transparent objects, a plain stage, sup- 

 ported on legs, with a mirror underneath, or a box like that 

 mentioned in page 53, wiU be all that is necessary. This 

 microscope has one principal advantage over that of any other 

 form of dissecting microscope yet contrived, viz., in the incli- 

 nation of the upper part of the eye-piece, so that the observer 

 is not required to bend his head, but to look straight forward, 

 whilst at the same time the object is seen in its natural 

 position. In fig. 236, 2, is shown a section of the compound 

 body, by which it may be easily seen how these two important 



