



88 THE NORTHERN MICROSCOPIST. 


iis APERTURE, SHULER. 
\ PARAGRAPH in our Notes and Queries column of last 
£1 month referred to the fact that this little piece of apparatus was 
exhibited by Mr. Collins to the members of the Royal Microscopi- 
cal Society. The plan was there discussed and the credit given to 
Dr. Royston Piggott, save the revival of the idea, with which we 
were credited. 
Now, as we described in the January issue (1882) of this Jour- 
nal, we had been using stops of blackened cardboard, with wide 
aperture objectives, to secure penetration, and it was owing to a 
conversation with Mr. Dancer, of Manchester, that the iris was 
constructed. Mr. Dancer produced, during the. conversation, a 
graduated diaphragm with a square aperture, made at least ten 
years since, but he thinks in 1870, to measure the extreme aperture 
to be given to the fixed diaphragm placed behind the back lens of 
all his objectives. 
When working thus, he discovered that small apertures gave 
greater penetration than wide ones, though he never seems to have 
done more than to note the fact. 
If we turn our attention to the literature of the Monthly Micros- 
copical Journal, we shall come across some curious statements, 
but, nevertheless, if a claim of securing penetration from wide angle 
objectives was made during the years 1869-1877, we shall doubt- 
less find it there. Upon searching, however, for Dr. Piggott’s 
claims, we come upon a host of matter which caused much dis- 
cussion at the time, but in no case can we find any special piece 
of apparatus devised for ridding wide angle lenses of what was then 
called their ‘‘ want of penetration.” 
It will be remembered by most of our readers that Dr. 
Piggott’s papers were to prove that nearly all objectives of that 
day (1870) were possessed of some residuary aberration, and that 
most structures were beaded. He also held that in order “ to de- 
fine some of the most minute lines of diatoms, fine definition is 
often sacrificed to too large aperture.” 
Dismissing the beaded structures, insect scales and diatom 
frustules in particular, with which the earlier volumes teem, and 
also the “ Aplanatic Searcher,” which was stated “to improve 
penetration, amplify magnifying power, and raise the objective 
somewhat further from the dangerous proximity to the covering 
glass,” we meet in M. M. J.,.iv. 300, with the Aberrameter, used 
for cutting off excentrical rays, and have followed it to a later 
volume, where it is described as made by Messrs. Beck. 
It seems to have been used for giving better definition to 
glasses possessing uncorrected margins, but not a word in the 

