INVERTEBRATA, CRYPTOaAMIA, MICROSCOPY, ETC. 1059 



removing his eye from the eye-piece ; while the quality of the illumi- 

 nation, in all its varieties, is such as is not surpassed by other more 

 or less special contrivances. Indeed, the general impression of those 

 who have used the instrument is that its illumination is more eflfective, 

 particularly in showing the delicate details of difficult objects, than any 

 other extant. 



" Lastly, and not least, the power of reading off on the sector the 

 angle of illumination used, whereby the effects of different angles of 

 illumination can be registered, resorted to again at pleasure with 

 certainty, or communicated to other observers, enabling them to do the 

 same, if provided with a similar instrument. 



" Perhaps I may be permitted to conclude this imperfect description 

 by mentioning what one who is well qualified to judge of the merits of 

 the instrument has commuuic:ited respecting it. He quaintly says, 

 ' I find but one fault with your Microscope, and that is, that it puts 

 me out of conceit with the using of any other.' " 



Although the above paper is dated 1858, it should be noted* that 

 the main features of novelty had been previously described by Mr. 

 Grubb, viz. in 1853, in the ' Proceedings ' of the Royal Irish Academy,f 

 and in his j)atent of 1854, J in both of which the graduated sectoral 

 arc is referred to. In the paper of 1853 Mr. Grubb said he had 

 mounted " a suitable illuminator on a vertical circular sector (nearly 

 a complete circumference), concentric with the focus ; this part of the 

 arrangement enables me to throw the beam on the object at all angles 

 of incidence, whether from beneath, as in the case of translucent, or 

 from above, in the case of opaque objects, and as the sector is 

 graduated, I have the power of observing or restoring any position at 

 pleasure." 



Thury-Nachet Traverse Substage. — This appears to be the next 

 in order of date, having been made by M. Nachet, on the suggestion 

 of M. Thury, in April 1855. 



The substage is shown in Fig. 127, separated from the Microscope. 

 It consists of two sector-bars C C equidistant from the object, mounted 

 parallel and attached to the main limb of the stand by screws behind 

 the square end G. These bars carry the condenser B above, and the 

 mirror A below, on a moving framework on which is a graduated 

 scale F for observing the degree of inclination. By means of a rack 

 and pinion (milled head D shown on tlie further side of figure) the 

 framework, carrying condenser and mirror, can bo moved concentri- 

 cally with the object, producing oblique illumination. The traversing 

 movement causes the toothed pinion H to turn in the rack J, and an 

 endless screw at the lower end of the same pinion (behind tlio milled 

 head E) works on the toothed wheel I attached to the mirror ; this auto- 

 matic motion keeps the reflected beam from the surface of the mirror 

 exactly in the axis of the condenser whilst the latter is being inclined 

 obliquely to the object. The mirror itself can bo adjusted by tho 

 milled head E, tho pinion through I being held ia position by 



♦ 'Engl. Mtch..' xxxi. (1880). t Vol. v. 



: See this .lourual, ii. (1879) p. 320. 



