114 CONSTRUCTION OF THE MICKOSCOPE. 



easily reached by tlie left hand, when the elbow is resting on the 

 table ; whilst the right hand finds the milled heads of the travers- 

 ing stage and of the secondary body in close proximity to each 

 other. The imperfection of the means of giving rotation to the 

 object, constitutes in this, as in Powell and Lealand's microscope, 

 a point of inferiority to Ross's ; the number of cases in which 

 such a movement is important, however, is by no means consi- 

 derable. On the other hand, the arrangement of the illuminating 

 apparatus in Smith and Beclf's Microscope, seems to the author 

 to present some decided advantages over tbat adopted by either 

 of the other makers ; and in point of general excellence of work- 

 manship, this instrument cannot be surpassed. 



Without any invidious comparisons, it may be safely said that 

 whoever desires to possess a, first-class Microscope, cannot do bet- 

 ter than select one of the three instruments last described ; the 

 excellence of the optical performance of the lenses supplied by 

 their respective makers, being so nearly on a par, that the choice 

 may be decided chiefly by the preference which the taste of the 

 purchaser, or the nature of the researches on which he may be 

 engaged, may lead him to entertain, for one or other of the plans 

 of construction which has now been brought under notice. 



40. Nachet's Binocular Microscope. — Since that remarkable 

 invention of Prof. Wheatstone, the Stereoscope, has led to a 

 general appreciation of the value of binocular vision, in conveying 

 to the mind a notion of the solid forms of bodies, various attempts 

 have been made to apply the same principle to the Microscope. 

 To any one who understands the principle of the Stereoscope, a 

 little consideration will make it obvious that this end might be 

 theoretically attained, by placing two microscope-bodies at such 

 an angle of inclination, that their respective object-glasses should 

 point to the same object, whilst their eye-pieces should be at the 

 ordinary distance of the right and left eyes from each other; but 

 this practical difficulty will obviously and necessarily arise, in 

 bringing the two microscopes into the requisite convergence, — 

 that the axes of the instruments cannot be approximated suffi- 

 ciently closely at their lower ends, unless the objectives employed 

 should be of a focus so long, that the value of such an instrument 

 would be extremely limited. It was early seen, therefore, that 

 the only feasible method would be to use but a single objective 

 for both bodies ; but to bisect the pencils of rays emerging from 

 this lens, so as to cause all those which have issued from the ob- 

 ject in such a direction as to pass through either half of it, to be 

 refracted into the body situated on that side ; so that the two 

 eyes, applied to the two eye-pieces respectively, shall receive 

 through the two halves of the objective, two magnified images 

 of the object differing from each other in perspective projection, 

 as if the object, actually enlarged to the dimensions of its image, 

 had been viewed by both eyes at once at a moderate distance. 



