A RADIAL SUBSTAGE ILLUMINATOR. 189 
While regretting that this firm could not have found a shorter title 
for this accessory, we must say it seems a useful method of pro- 
ducing obliquity of light. It is shown in Fig. 33. 
This apparatus has been constructed for the purpose of i increasing 
the resolving property of high power objectives by causing still 
more oblique pencils to impinge upon the object than can be 
obtained by most methods. The arrangement consists first of an 
arc-plece, fixed below the stage, radial to an imaginary line drawn 
through the axis of the microscope objective, in the same piane 
with the object. On this an achromatic condenser of special 
construction is made to travel, thus keeping the rays of light on the 
object during its entire traversing. These rays converge and 
terminate in a focus through the front lens, in a highly concen- 
trated form. The condenser i is illuminated by a rectangular prism, 
for condensing light into the achromatic combination. ‘The next 
part consists of a second arc-piece placed at right angles to the 
former one; this also carries a similar achromatic condenser and 
illuminating prism, which move radial to the centre. Both these 
arc-pieces are so divided that each pencil of light can be projected 
at a similar angle, and previous results can always be recorded in 
the same way. Difficult test objects are readily resolved, especially 
such diatoms as have rectangular striz or markings, W ith a ¢-Inch 
objective, the diatom Navicula rhomboides is easily resolved into 
squares. ‘The markings on / angulatum by the same means is 
