256 Dr. Gorino- on Chevaliet-'s 



o 



of eye-pieces. The double motion given to the body of a 

 microscope is, in our idea, the defect of all those hitherto con- 

 structed ; for, as the optical part should remain perfectly centred 

 with the mirror and the diaphragms, it is evident that the 

 least derangement of it from this position must destroy the 

 perfection of the image : the stage then only should move* 

 without affecting the diaphragms or the mirror, and in this we 



* This is the theoretical view of the case, the practical one is different, 

 as frequently happens. Opaque objects are not affected at all by the 

 eccentricity of the axis of the body : nor can I recollect that I ever felt 

 any particular inconvenience from the motion of the optical part, even 

 with transparent subjects, unless it was thrown very much indeed out 

 of the axis of the illuminating mirror. On the other hand, it is notorious 

 that living aquatic insects and animalcules are the most popular and 

 entertaining objects which microscopes can show. These are, for the 

 most part, abundantly restless ; and if the stage on which they are placed 

 has any motion, their natural unquietness is so much exasperated, that 

 it becomes almost impossible to get a good observation of them at all. I 

 once set to work at making some drawings of a variety of new and ori- 

 ginal objects of this class (which, I trust, will one day be published) with 

 a microscope having all the requisite motions applied to its stage, and am 

 confident that I had thrice the labour fairly appropriate to the execution 

 of my task from this oversight. The very tremor produced by the transi- 

 tion of a carriage in the street, is frequently sufficient to unsettle live 

 objects when disposed to be still and quiet, and put them in a fidget for 

 a quarter of an hour. It is very unfortunate that the mountings of 

 optical instruments are made in general by mere mechanics, who seldom 

 or never observe with them, and consequently know not the exigencies 

 which occur in practice. It is still more unfortunate, that in the science 

 of fitting up microscopes, an ounce of a man's own wit is worth about a 

 ton of his neighbour's. Was it not that I 

 dislike to verify this adage myself, I should 

 recommend the following motion to be ap- 

 plied to the body : — let the socket of the 

 arm which carries it have a smooth rotatory 

 motion on the head of the bar in the usual 

 way, conjoined with another horizontal one 

 produced by rack work attached to the said 

 socket. Let the pinion which belongs to the 

 latter movement be made very strong, so 

 that a lever about three inches long may 

 project from its centre: this is to be held 

 in the hand, the thumb and index finger ope- 

 rating on the rackwork, while the two little 

 fingers give a rotatoiy motion by working the lever end. This rapid 

 double motion is here completely under the command of one hand, while 

 the other is at liberty to adjust the focus. I know of nothing better 

 for general purjDOses, or in particular for following the motions of live 

 insects, &c. ; but when only inanimate ones are to be the subject of mi- 

 croscopical study, I prefer the motion of the stage, for the reasons stated 

 by Mons. C. 



