OBJECTIVES AND OCULARS. 
65 
will be found that, for the above reason, the No. 6 pro¬ 
jection ocular is difficult to use except with a short 
camera, but, so far as we can discover, a given magnifi¬ 
cation can be obtained as successfully with the No. 6 
ocular and a 12 inch camera as with the No. 3 and a 
24 inch stretch. The camera of Dr. Van Heurck does 
not stretch beyond about 20 inches, yet no one is likely 
to find fault with his results on this score. 
A moderate battery of glasses might be as follows: 
A 70 mm. Zeiss; a one inch, by any good maker, 
achromatic; a quarter-inch, such as the apo. 6 mm.; and 
an immersion twelfth. The following might be con¬ 
sidered a complete armament: 70, 35, 24, 12, 6, 3, 
and 2 millimetres, or their equivalents in English 
measure. The 70 and 35 mm. glasses may be 
“ Planars ” of Zeiss. The two finest and most useful 
glasses the writer has ever possessed are the 12 and the 
3 mm. apochromatics, and next to these the aa achro¬ 
matic for use without ocular. Of oculars, the No. 8 of 
Zeiss is very useful, but ordinary achromatic lenses 
begin to break down even visually under this ocular, so 
that for these lenses it is well to have an ocular of 
lower power. The projection ocular may be chosen 
according to the length of tube and of camera intended 
to be used. In view of the facts stated with regard to 
the use of orthochromatic plates with “ uncorrected ” 
lenses, it may be pointed out that very fine results, 
where the greatest apertures are not required, may be 
obtained with good achromatic glasses, provided colour- 
correct plates are used with the addition of the “screen ” 
