48 THE NORTHERN MICROSCOPIST. 

only shown by immersion objectives having more than 82° angle in 
glass. 
IMPROVEMENTS IN PHOTO-MICROGRAPHY.—I have great pleasure 
in sending enclosed sketch of a piece of apparatus (Fig. 4) which I 

Fig. 4. 
have no doubt will facilitate one of the most difficult operations in 
Photo-micrography, viz., that of rapidly finding the difference between 
the chemical and visual foci which micro objectives possess, more or 
less (especially the lower powers). You will see that it is a revolving 
shutter (Fig. 5) with a diagonal opening (#) to be fixed just in front of 

Hagy 25. 
the dark slide.™ Six different exposures may be made on one plate, 
each with a slightly different amount of adjustment of fine screw, 
