76 PRACTICAL PHOTO-MICROGKAPHY. 



Thus : A lens of one inch focus gives at about 10 inches up the 

 tube a magnification of 10 diameters ; by whatever means we try 

 to get from that lens a magnification of 100 diameters projected 

 as a real image, it must be a good lens if it stands that strain with- 

 out breaking down in definition or corrections. In fact none but 

 the very best objectives will tolerate any such strain. By dint of 

 exceedingly skillful manipulation such as only a few men can 

 claim to have acquired, we have known specially fine objectives 

 stand a stretch of their powers even greater than this, but the 

 writer's own attempts in this direction have always been utter 

 failures, and in the majority of cases six or seven times the 

 initial power is ample to produce a faltering of the lens' 

 capabilities. And this holds good whatever be the means 

 adopted for increasing the magnifying power of the objective, 

 whether long stretch of camera or high eye-piecing, or the two 

 combined. Thus Zeiss makes two projection oculars for one 

 series of lenses ; the first ocular increases the magnification by 

 " three times " and may be used without difficulty up to a 

 stretch of about 30 inches from ocular to sensitive plate (giving 

 a power about nine times the "initial power") ; while the 

 higher power projection ocular magnifying " six times " will 

 at the same stretch break down the finest objectives, unless the 

 skill of the operator be very great indeed, greater than the 

 writer can claim, certainly. 



"Stopping down" Objectives. It is common to find at the 

 back of objectives a cell or diaphragm constricting more or 

 less the light-way from the objective to the eye or to the plate. 

 So long as the " stops " do not cut off any pencils of light that 

 otherwise would reach the plate or the retina no harm is done ; 

 but if the stops are used as a supposed means of reducing 

 aberrations or incorrectnesses of the objective, a great deal 

 of harm is done and a very foolish mistake made. By the 

 time that the pencils of light have passed through the objective 

 the mischief is done, if it is done at all, and a stop behind 

 the objective may hide, but cannot possibly correct any 

 errors. Whatever " stopping "is to be done should be done 

 in front of the objective, that is to say in the condenser, or, 

 failing a condenser, in the substage. If, therefore, the reader 



