102 PHOTO-MICROGRAPHY 



another objection to the use of those with small aplanatic cones— they are so extremely 

 seMsitive to slight diffeTe7ices in centreing. 



In most microscopes this centreing arrangement is provided for in the substage 

 itself, as before explained, but the firm of Zeiss (and, we believe, a few others), do not 

 do so, for their achromatic condenser is fitted with a centreing collar of its own, 

 which with the optical portion of the condenser forms one mass, to be dropped into 

 the substage itself. "We show in Fig. 55 their arrangement. It is unnecessarily 

 heavy and not optically devised so well as it might be, for whilst possessing a N. A. 

 of -98, its aplanatic cone is only '65, far too small we think : an opinion shared by 

 many others (see Carpenter on the Microscope, Edition 1891, page 263). 



Underneath the condenser some arrangement always exists to reduce its aperture. 

 This is done either by the use of the iris diaphragm (see D in Fig. 58), or by a 

 metallic plate perforated by several holes of different sizes. This plate revolves so 

 truly that the centre of each hole is in the axis of the microscope ; so, too, with the 

 iris diaphragm, for when that is shut down its aperture should be central in the same 

 manner. Some microscopists prefer the wheel diaphragm, as it is called, and others 

 the iris ; both have their advantage and disadvantage. The objection to the use of 

 the iris diaphragm is the fact that, in the older forms there was no means to show to 

 what extent it had been closed, so that when using the specimen again the process of 

 ascertaining what sized aperture gave the best results had to be repeated de novo ; 

 whereas, if the wheel diaphragm had been used the size of the hole giving the best 

 definition could have been noted for future reference. In the Zeiss and other modern 

 microscopes this is got over by graduating the arc through which the handle of the 

 iris is made to turn. Underneath the diaphragm, or in some cases immediately 

 above it, is usually some arrangement to carry coloured glasses ; obliquely or other- 

 wise cut diaphragms ; and some suitable arrangement for obtaining annular light and 

 dark ground illumination. =^ This latter usually consists of a ring of brass having 

 three radii passing from the periphery to the centre, such centre being rigid and of 

 any size desired. Seeing, however, that the size of this stop has to be varied with 

 each lens that is used, it is very obvious a large number of these stop diaphragms 

 must be employed by the microscopist, and inasmuch as they are rather expensive, it 

 becomes troublesome. It was to get over this difficulty we designed the arrangement 

 shown in Fig. 48 (page 95). Several sizes and several shapes are supplied with this little 

 piece of apparatus, so that the microscopist can readily suit each lens as he thinks fit. 



* So far as relates to the coloured glasses, personally we prefer the use of a strip held by a Bunsen holder apart 

 and so having no connection with the microscope ; because its removal does not shake the instrument. It is shown in 

 Fig. 24b (page 54). 



