LOW-POWER WORK 17 



the centre of the field, it is true, will be seen on the ground glass 

 at P, well and clearly defined, but the portion surrounding the centre and extending 

 to the edges will be fuzzy and useless as shown with quite a small specimen in Fig. 6, 

 Plate I. If we had been photographing the spider shown in Fig. 3, Plate I., the legs 

 would have been fuzzy whilst the body was sharp and well defined. 



An ordinary photographic lens is the next thing that suggests itself, but here we 

 are met with a two-fold difficulty : First, suppose, for instance, we were to use a 4 or 

 5 inch rapid rectilinear, we should require so very great an extension of camera that 

 it would be impossible to reach the focusing screw of the support S without the 

 assistance of a "long arm " such as is used with an astronomical telescope. It is not 

 advisable to make the apparatus more cumbersome than is necessary, and so the only 

 escape from the difficulty is to have a lens of shorter focus. 



We must point out here, as perhaps the most suitable place, that the accurate and 

 final focusing should always be done by moving the object rather than by moving the 

 ground glass, as is usual when taking an ordinary photograph. The reason is not far 

 to seek, but to explain it intelligibly we must for a moment enter into the optical 

 aspect of the situation ; it is worth a moment's consideration, as it at the same time 

 explains the second difficulty connected with using ordinary photographic lenses for 

 photo-micrography, which should not be omitted. 



When photographing in the ordinary manner — say, for instance, taking a view of 

 a landscape — the rays from distant objects impinge on the lens, usually in more or 

 less parallel bundles, and the ground glass is placed at the short conjugate focus, the 

 lens being constructed by the optician with that idea in view. But in making an 

 enlargement with a photographic lens the conditions are exactly the reverse, for then 

 the short conjugate focus is between the lens and the object, and the long one 

 between the lens and the ground glass screen. It will be seen therefore, that the 

 lens is constructed for one purpose and used for another, hence it is not difficult to 

 understand the absence of critical perfection in the resulting image. It has been 

 suggested, to get over this difficulty, that the lens should, when used for photo- 

 micrography, be turned the opposite way, and this improves the resulting definition 

 with almost any lens excepting a rapid rectilinear, but with this type we have 

 never been able to find any pronounced improvement by so doing, and, indeed, on 

 considering its optical construction we should never suppose that it would. 



Another class of lens has been suggested for photo-micrography, and we have 

 often used it where the object is large and the required magnification is small : we 

 refer to a diminutive portrait lens of about 3-inch focus built on the Petzval principle. 



