Phenomena of the Podura Test. 171 



The opticians were fairly credited with the most honest inten- 

 tions when they coustrncted the glasses to show this standard and. 

 no other. They naturally desired to display microscopic power in 

 its most approved and effective form as a siinimum honum of art. 

 The exclamation markings survived as a test for forty years. I am 

 fortunate to possess an ancient scale of the Test Podura and one 

 of Pritchard's doublets l-80th of an inch focal length, capable of 

 showing these veritable markings. Why the resolution of these 

 markings into much more minute structure by the glasses of to-day, 

 when these same markings could be seen forty years ago with a 

 doublet, should be so much doubted, appears almost unaccountable ; 

 and offers an interesting phase in the science of microscopy. No 

 such a thing happened when the lined objects, such as diatoms, gave 

 way to squares and dots ; more complete resolution was hailed with 

 satisfaction as a sign of progress. Science is not without many 

 examples of outcry against, and disbelief in, new truths. 



I attach very much importance to the phenomena displayed by 

 the Podura Test-scale for several reasons : — 



1. They are a severe test for chromatic and spherical errors. 



2. An objective and eye-piece which is competent to thoroughly 

 resolve this test from its j)rimary markings into its secondary and 

 tertiary forms may be pronounced super- excellent. 



3. The scale is eminently fitted also to display the amount of 

 penetration of an objective, as it possesses structures which are 

 placed in two different planes ; the one nearer and the other farther 

 from the objective. 



4. The scale gives a new lesson in microscopy, teaching the 

 observer how to deepen or regulate the planes of focal vision inde- 

 pendently of mere focussing. 



Indeed, objectives can be so adjusted and compensated as to 

 enable the observer to view either the upper or the lower structure, 

 or both structures at once. 



5. The scale is further valuable as embracing almost every form 

 of difficult definition : lines, points, beads or molecules, and brilliant 

 disks of light, as well as the colours due to films of various thick- 

 ness as shown in " Newton's Eings." 



THE SHADOW PHENOMENA. 



The standard (?) markings of forty years' pedigree may be seen 

 with a good inch objective, or with a powerful doublet. A startling 

 fact is here disclosed. Onlij a vrij small angular aperture is re- 

 quisite to display them. The markings (! ! !) are therefore inde- 

 •^endent of large aperture. If we could imagine a Podura scale 1000 

 times larger than usual, there does not seem the smallest reason to 

 doubt that we could see these markings with the eye alone at a 



