170 PEACTICAL PHOTO-MICBOGKAPHY 



In actual use all these lens systems are subject to practically 

 the same Working conditions, and it is therefore proposed to 

 describe the method of adjusting the complete lens system 

 already described as made by Messrs. Zeiss ; these meet almost 

 every need, are comparatively inexpensive, and are available 

 for use with powers from the lowest to the highest. 



In conjunction with these lenses an iris-diaphragm on a 

 separate stand is usually supplied, and in any case, whatever 

 form of collecting -lens is used, such an addition is almost a 

 necessity. A less expensive substitute is a series of metal plates, 

 with holes of various sizes in them, which may be supported 

 in any suitable carrier and which are so arranged that they 

 can be easily changed as required. The first point is to ensure 

 that this iris- diaphragm or its equivalent is accurately centred 

 to the optical axis of the microscope, when the latter is in 

 the horizontal position, and is in alignment with the centre 

 of the camera back. To be quite certain that this is the 

 case, it must be observed in relation to the sub-stage iris 

 on the microscope. In some instruments there are means 

 of de-centreing this iris, the rack-work that enables the 

 de-centration to be effected having a catch which indicates 

 to the worker when centration is correct. With the usual 

 English form centration of the iris would be effected by 

 means of the centreing-screws on the sub-stage. The operation 

 may be performed by placing a low-power objective in the 

 microscope, inserting an ocular, and viewing the image of the 

 diaphragm directly. Any necessary alteration in the adjust- 

 ment may then be accomplished by means of the screws on the 

 sub-stage already mentioned. The ocular is next removed, 

 and in its place a cap, which is perforated at its centre with a 

 small hole less than one millimetre in diameter, is substituted. 

 On observing the source of light through this eye-piece cap 

 and the closed iris -diaphragm of the microscope sub-stage, 

 it will at once be seen whether the illuminant is in correct 

 alignment, as unless it is, no light will pass through to the eye. 

 The illuminant having been adjusted centrally, the subsidiary 

 diaphragm first mentioned may now be interposed between the 

 sub-stage of the microscope and the illuminant. It facilitates 

 any future adjustment if these extra fittings are carried on a 

 triangular metal bar or geometric slide, as already described ; 



