66 
PRACTICAL PHOTO-MICROGRAPHY. 
when necessary. This greatly lessens the pecuniary 
outlay necessary to produce the finest results usually 
required for medical, biological, or mineralogical work. 
We have lately had the opportunity of working with 
several objectives sold by Messrs. Watson and Sons 
under the name of “ Idoloscopic.” The focal lengths 
of those tried were stated to be, and actually were, 25, 
12, 6, and 4 millimetres ; the front combinations were 
remarkably, almost startlingly, large, and the objectives 
worked admirably when the back combination was half 
to two-thirds full (N. A. -3, *45, -8, -g). We photo¬ 
graphed with these lenses all the usual test objects for 
such apertures, and found the results excellent. The 
rapidity of action, as might be expected from the large 
area of the component combinations, was very marked ; 
the lenses worked probably twice as fast as any other 
lenses we have used under similar conditions. There 
was certainly a noticeable amount of colour in the 
images, but the definition was admirable. Having 
used these objectives with Zeiss Projection Oculars as 
well as oculars understood to be intended for use with 
these lenses, we believe that for the photo-micrographic 
purposes these objectives rank next to true apochro- 
matics, while the prices (see Appendix) ought to place 
them well “within the reach of all.” 
The objectives of this series were intended for use 
with a ten-inch tube, and had no correction-collars ; we 
found one of them worked better with a tube about 
8-§- inches long. 
The oculars examined along with these “ Holo- 
