334 Trmisactions of the [Sess. 



apparatus invented and constructed by himself — viz., a self-center- 

 ing turntable and an improved form of centering nosepiece. The 

 following are Mr Frazer's remarks on these '.-^ 



ON A SIMPLE FORM OF SELF-CENTERING TURNTABLE 

 FOR RINGING MICROSCOPIC SPECIMENS. 



The construction of a turntable is so well known, that it is only 

 necessary to point out wherein this appliance differs from other 

 machines of the same sort. It is made (1) much larger and 

 heavier than usual, so that slides which have the specimen 

 mounted not in the middle of the slide wiU not project beyond 

 the edge of the disc when being ringed ; (2) the springs are made 

 with a special form of " washer," so that these (the springs) may 

 be turned freely in any direction ; (3) the turntable is provided 

 with a simple arrangement, consisting of three screws, which are 

 placed in such positions upon the table that slides either of one 

 inch or one and a half inch, if placed against them, will be ac- 

 curately centred ; and the screws are also so arranged that when 

 it is desired to use the turntable as a non-centering one, the 

 screws may be depressed below the surface of the table.-*- 



ON AN IMPROVED FORM OF CENTERING NOSEPIECE, FOR USE 

 AYITH BROOKE'S DOUBLE NOSEPIECE FOR MICROSCOPE. 



The use of the double or triple nosepiece has become very 

 general in microscopical investigation, and especially so in the 

 departments of pathology and physiology ; but the continued use 

 of this appliance has shown that it labours under two defects. 

 In the first place, when the nosepiece is moved in the usual way, 

 and one objective put in place of another, it seldom happens that 

 an object which was in the focus of one power is also in the focus 

 of the other ; and, as a consequence, the operation of refocusing 

 must be performed. This defect may be remedied by making the 

 sides of the nosepiece which hold the objectives of unequal lengths, 

 or by putting an adapter in either side, and so correcting for the 

 , difference of adjustment for focus. When this correction has been 

 made, the convenience of the nosepiece is much increased ; but the 

 error of want of concentricity may still remain — i.e.^ a particular 



^ In the disc five holes are drilled : one of these is at a distance of I^ in. 

 from the centre ; and of the others, two are placed in a line dra-wn parallel 

 to a line -which joins the first-mentioned screw and the centre, and at a 

 distance of half an inch from it ; and the other two are placed in a line 

 which is three-quarters of an inch distant from the line of the first screw and 

 the centre, and also parallel to it. 



