15 



The finest and most resj)lendent definition of the 

 diatoms may certainly be seen with the immersion lens and 

 a l-16th, which it converts into a l-20th objective. The 

 structure of the P. formosum has been an object of care- 

 ful study for many years, as one of the easiest forms for 

 definition. Separated spherules generally characterise this 

 diatom. The beading of this object is brilliant in the extreme, 

 never grey. The exquisite beauty of these minute gems of nature 

 rival the most glorious tints of the diamond, ruby, or sapphire ; 

 but a power of 7000 diameters begins to develop shadow and 

 haze. Betiveen the spaces of the upper beading another struc- 

 ture is discernible, but whether the interspaces are crossed 

 by a deeper set of beading or the uj^per set are superimposed 

 upon the lower I cannot at present decide, but I strongly 

 incline to the latter supposition. 



P. strigosum. Here the upper set appear to hide a 

 parallel lower set of beads, like row upon row of cannon shot. 

 But always do I perceive the two sets of different colours, one 

 row pink-red, the intervening row violet or blue -, probably 

 the colours are produced by the dispersion of position, and 

 may be good evidence of the sets existing in different planes. 



P. hippocampus. Similar phenomena are observed. Rows 

 of beading appear to cross in different planes at right angles 

 to each other. 



A severe test is the appearance of minute hairs 1 -50,000th 

 of an inch diameter. A fine definition shows a hair to bear 

 two black borders and a central line of light, with scarcely any 

 penumbra under the l-16th and immersion lens. Hairs of 

 antennae of male gnat were employed. 



Glittering particles of gold leaf. Some of these may be 

 found l-50,000th of an inch in diameter ; brilliant illumination, 

 if the corrections be not good, shows four to five diffi-action 

 rings. I have seen them diminish to one. 



Crystalline surface of metal recently broken. The glare is 

 universal unless the aberration is very finely corrected. 



On Testing the Magnifying Power. 



Sir John Herschel's definition of the power of a lens, 

 as unity divided by the focal length, is, perhaps, the best 

 that can be employed. But the real magnifying power 

 varies with different persons, as with short-sighted and 

 long-sighted observers. As a standard, the power of a 

 lens forcing an image to the eye at ten inches may be 

 10 -f- focal length. On this principle an inch objective 

 should form an image ten times larger than the object at a 



