132 PKACTICAL PHOTO-MICBOGKAPHY 



is immediately underneath the position of the fine-adjustment. 

 Between the fine- adjustment and this double- grooved pulley 

 an endless silk cord passes which has very slight tension on it, 

 and is only sufficiently tight to ensure movement of the fine- 

 adjustment when any motion is imparted to the pulley. As 

 already explained, a very considerable degree of slackness of 

 the silk cord is permissible and, if it is well waxed, the 

 tension on the fine-adjustment is reduced to a minimum. The 

 arrangement further imparts a very smooth and regular motion 

 to the fine- adjustment on rotating either of the milled heads 

 on the long steel rod, and it has the additional advantage 

 that, owing to the connecting- cord passing up through the 

 hollow centre on which the tail-piece revolves, no part of the 

 focussing apparatus requires to be detached if the microscope 

 has for any purpose to be swung out. 



The second and alternative arrangement for focussing is the 

 method already described as attached to the Zeiss photo-micro- 

 graphic apparatus (Fig. 48). In actual working it does not 

 appear to have any advantage over the method above described, 

 although perhaps the response of the fine-adjustment to any 

 rotation of the focussing-rod is more immediate in the case of 

 the Zeiss method, whereas with any arrangement in which there 

 are cords there is always some slight stretch of the cord itself on 

 reversing the direction of rotation which has to be allowed for, 

 and which happens before the fine-adjustment motion comes 

 into play. The Zeiss method has the distinct disadvantage, 

 that if any adjustment of the microscope becomes necessary, 

 and the tail-piece is revolved to allow this to be effected, the 

 focussing- gear has to be detached, and this is of course not 

 necessary with the method first described. The whole apparatus 

 may be used for photographing with low powers or ordinary 

 photographic lenses. Fig. 47 & shows it so arranged, the 

 microscope and its table being removed. 



This apparatus in actual work has been found to fulfil every 

 possible requirement. It is rigid in the extreme, and any 

 vibration or tremor conveyed to it does not affect the results, 

 as the apparatus responds as a whole. It is of necessity some- 

 what expensive, and for many purposes unnecessarily elaborate 

 to build up in the way described and illustrated. It is, however, 

 important to recognise that it is designed entirely on the 



