LOW-POWER WORK 19 



lion was initially smaller and within the "covering power" of the 50 mm., so that 

 lens could be employed for that object. 



It will be seen now, it is hoped, what an important point this " covering power " is 

 when buying a lens. We have often been asked the best " all round " Planar to 

 buy ; but it is obvious we cannot say unless the usual size of object be given that it 

 is desired to photograph. 



It is needless to add the focal lengths of these admirable lenses differ and so do 

 their covering power, but all details are given in either Ross or Zeiss's catalogue. 

 The 50 mm. at work is also shown in Figs, i, 3, 4, 5, and 7, Plate 11. 



The camera may be of any size, but as before stated we have usually found the 

 quarter plate sufficient for small subjects, when the magnification required is small. 

 In our own special case we have both sizes made interchangeable, which is at least a 

 convenient arrangement. When using either form it is of great service to have an 

 auxiliary front which drops on to the camera where the lens usually fits into the 

 camera. By this expedient we are enabled to increase our camera length another 

 6 or 8 inches for the quarter plate, and 2 to 3 feet for the half plate. As will be 

 seen hereafter, a special arrangement is made for these purposes in the excellent 

 apparatus made b}^ several firms. 



Having thus described the apparatus, only one thing remains before taking the 

 photograph ; that is to select a suitably sensitised plate. We have tried most of the 

 brands in the market, but owing to the fact that so few are isochromatic, the selection 

 practically rests with three or four makers. The object of using isochromatic plates 

 will be explained hereafter, and we believe that, as far as isochromatism is concerned, 

 the Cadett plate is the most perfect, but it is very slow, which is a great objection to 

 its use. Not that the exposure need be very long with this class of photo- 

 micrography, but we must never lose sight of the fact that the rays emanating from 

 the limelight being focussed on the object, will very readily make it melt, and 

 therefore it is obvious, within prescribed limits, the quicker the plate the more safety 

 to the specimen. It is quite true that a water-bath can be interposed between the 

 condenser and the object, but it is very inconvenient to use with this lower power 

 work, for the water quickly begins to generate bubbles of steam, which, should they 

 adhere to the sides of the trough, will cause all manner of markings in the resulting 

 picture. We have rarely had the accident happen of melting a slide, although we 

 admit that at times they have become very warm, but if a prolonged exposure had 

 been added to the time necessarily occupied in focusing the object, it is highly 

 probable such an accident would have occurred more frequently. 



