70 SECOXD-CLASS MTCROSCOPES. 



ratus not suitable to being received by the object-carrier, which 

 can be easily slipped away from beneath the ivory knobs, thus 

 leaving the stage free. To the under side of the stage is firmly 

 pivoted a broad bar, into which is screwed a short sprung 

 tube, that is exactly concentric with the optic axis of the instru- 

 ment when the bar (which is shown turned-away in the figure) is 

 pushed beneath the stage until checked by a firm stop ; and as this 

 bar is composed of two pieces, held together by a pair of screws 

 working through slots, the centering of the tube may be precisely 

 readjusted if it should at any time become faulty. Into this tube 

 may be inserted another that carries either (1) a Diaphragm, which 

 can be slid up and down, so as to vary the proportion of the pencil 

 of convergent rays thrown upwards by the mirror ; (2) a Pola- 

 rizing prism ; (3) a Ground-glass for diffusing the light, which 

 may be either plane or a plano-convex lens, ground on its flat side 

 which is directed upw r ards ; and (4) a Glass Cone, having its apex 

 pointing downwards, and a large black spot in the centre of its 

 base which is directed towards the object ; this serves the same 

 purpose as the Paraboloid now commonly applied to English 

 Microscopes (§ 85). Lastly, the Mirror is attached to a stem 

 which is so jointed as to enable it to reflect rays of very great 

 obliquity. — To those who wish a compact instrument of great com- 

 pleteness and capability, which may be worked advantageously 

 even with high powers (for which an Achromatic condenser might 

 easily be added if desired), the Author can strongly recommend 

 this Microscope, especially when furnished with MM. Nachet's 

 Stereo-pseudoscopic arrangement (§ 29). The rotatory movement 

 of the Stage has all the advantages which are only obtained at a 

 great increase of cost in First-class instruments ; and it is so exact 

 as to answer equally well for all the purposes which this rotation is 

 specially fitted to serve (§§ 48, 52). The traversing movement of 

 the Object-holder is in some respects (especially for following living 

 objects) decidedly superior to that of any Mechanical Stage ; and 

 those who have become accustomed to its use will seldom feel the 

 need of the latter more costly appliance. The Sub-stage fitting is 

 so arranged as to carry the most needful Accessories, without either 

 interfering with extremely oblique illumination (as is done by the 

 tube which is screwed into the aperture of the stage of most 

 English Students' Microscopes), or requiring any complicated and 

 therefore costly provisions for the exact centering of its fittings 

 with the optic axis of the instrument. And the manner in 

 Avhich the Mirror is mounted gives it a remarkable range of posi- 

 tion. — The Objectives ordinarily supplied with this instrument by 

 MM. Nachet are of excellent quality, and are quite adequate for 

 the ordinary purposes of scientific investigation ; but for the sake 

 of purchasers who may prefer Objectives of English or American 

 make, MM. Nachet now provide it with the universal screw.* 

 - The price of this instrument, with the Accessories above enumerated, 



