126 Transadions of the [S^r^ro^^' 



here diffi-action lines are multiplied, and the visibihty of the lines is 

 a function of the breadth of the groove ploughed in the glass, — the 

 depth to which it is cut, — the sectional shape of the groove itself, — 

 and the direction and character of the illumination employed. All 

 these variable conditions in some measure detract from the fixed 

 value of this test. It ought to depend upon the uniformity of 

 standard conditions. The Acus, as shown by Mr. Powell, is a sharper 

 and safer test. 



A valuable series of pa]^)ers by Mr. Wenham, "On the Con- 

 struction of Object-glasses for the Microscope," will be found in the 

 'Monthly Microscopical Journal' for 1869. These papers were 

 considered by the author to be communications to our Society; but, 

 owing to then- nature and arrangement, the formal reading of them 

 was, at his request, dispensed with. It is acknowledged by all 

 opticians that Mr. Lister's introduction of the ti'iph-hack, i. e. the 

 innermost of the three combinations, has proved " the grandest step " 

 towards the perfection of the compound objective. Mr. Wenham 

 was no doubt the first to connect with this new back a lens nearly 

 hemispherical, instead of the cemented triple front. Mr. Wray, 

 following closely but independently in his wake, used what he 

 calls " the kettledram system for front " so early as 1851. It gives 

 confidence to observers when they find that able men are driven 

 by the necessities of the case to work by almost the same rules. 

 Accordingly I received with pleasure a note from Mr. Wray, in 

 which he says, " Judge of my suq^rise when I read Mr. Wenham's 

 first paper, and found he was using a form very nearly similar to 

 my own." Mr. Wenham points out very forcibly that the single 

 front has the advantage of facility of construction, and also gives 

 command over any required extent both of aperture and power. 

 When we find the diameter of a single front to be only the yV^h of 

 an inch, Mr. Wenham may well say, " The difficulty if not impos- 

 sibility of constructing a tri'ple of such almost invisible atoms of 

 glass may be imagined." 



Despairing of taking up the subject again practically, Mr. Wenham 

 leaves, as a legacy to opticians, an improvement even on the single 

 front, and proposes to construct the higher powers with two 

 single lenses in front, of equal radius. If these are set in contact, 

 the magnifying power wiU be nearly as their sum ; they may there- 

 fore be made of double the radius, and consequently nearly twice the 

 diameter, which, of course, would lessen the practical difficulty of 

 working a ^Vth, and enable us to go even beyond this power. A 

 partial experiment with a ifh, having this " doublet " front, has 

 proved that perfect correction for colour is the result. Our leading 

 opticians will no doubt thankfully avail themselves of these sug- 

 gestions of an amateur labourer whose masterly knowledge of the 

 subject and almost unrivalled practical skill fit him to speak ex' 



