246 



HAItDWICKE'S SCIENCE-GOSSIP. 



never equal even a moderately good compound ob- 

 jective of modern manufacture. The severest test 

 objects of fifty years ago were the scales of insects, 

 particularly those of Lepisma and Podura. Mr. 

 Pritchard cautions observers to be sure of 

 the kind of scale they are about to use as a test, 

 as he truly observes some scales of Lepisma or 

 Podura are easily revolved, while others tax the 

 powers of the best objectives ; this caution applies 

 equally well to the present time. An objective 

 may be guaranteed to resolve, say, Pleurosigma 

 angulatum, but unless we know whether it is 

 the robust form, such as those mounted by Moller, 

 or the commoner and more delicate form we find on 

 our own coast, we are still in ignorance of its resol- 

 ving power. The following is a list of objects used 

 as tests for jewelled microscopes and engiscopes. 

 Eor penetration, first section (easy), scales of Petro- 

 bins maritimus, Lepisma saccharina ; second section 

 (standard), " Feathers " oiMorpJw menelaus, Alucita 

 pentadactyla, A. Jiexadactyla (from body), Lyccena 

 ctrgus, Tinea vestianella (from under-side of the 

 wing); third section (difficult), "Feathers" of 

 Pieris brassica, scales of Podura plumbea. For 

 definition, hair of Mouse, ditto of Bat, leaf of 

 Hypnum species, spotted scales of Lyctena argus. 



The Lepisma scale seems to have been very fairly 

 shown, judging from the figure given of it. The 

 scale of Morpho menelaus, a somewhat more difficult 

 test, displayed two sets of markings. 



The appearance of the scales of Alucita, or plumed 

 moths, is thus described : — " The scales should be 

 taken from the body of the insect, and not from the 

 plumes ; their breadth is generally greater than their 

 length, and their form is never symmetrical. They 

 are transparent, and about y^ of an inch in length. 

 The scale is often partially covered by a delicate, 

 uneven membraneous film, which obliterates the lines 

 on those parts. The longitudinal lines are not dif- 

 ficult to resolve, but their proximity is such that 

 they require a considerable power, and careful 

 illumination to separate them distinctly. They are 

 elegant microscopic objects, but rather scarce." 



The favourite test appears to have been the 

 tufted scale of the cabbage-butterfly, Pieris brassica. 

 The genuine test scale is of a pale-yellow colour, 

 and very transparent. This object requires the 

 light to be more oblique than any other of the lined 

 kind, and was seldom to be made out, excepting 

 when the magnifier was much out of the axis of the 

 perforation. . " I [Mr. Pritchardj have seen them 

 with a single jewel lens of only T V of an inch 

 focus." Other markings were thought to be dis- 

 tinguishable on the scales, viz., a series of oblique 

 lines running in opposite directions. " They are 

 always fainter than the others, and both sets are 

 never seen together. I have seldom seen them by 

 daylight, and even with artificial light they are 

 not easily resolved." ti 



These lines have no real existence, as a reference 

 to more modern figures will show. Quekett gives 

 a very good figure ( x 500 diameters) of this scale 

 in his "Practical Treatise on the Microscope," 

 1848, plate 6, fig. 2. We come now to a test 

 which still puzzles the observer, and even with 

 the best objectives and most perfect appliances 

 the real nature of the markings has not yet been 

 satisfactorily demonstrated. The Podura scale 

 was the ne plus ultra of tests for the objectives of 

 engiscopes aud jewel lenses, and, as a description of 

 what the microscopes of fifty years ago would do in 

 this object, we will quote Mr. Pritchard's account 

 of the appearance of the markiugs as seen under 

 one of the best instruments of the period: — "I 

 have before remarked that on the discovery of any 

 more difficult object than what is already known, 

 an improvement of the microscope has soon fol- 

 lowed. This was strikingly exemplified in the 

 discovery of the lines on the scales of this insect. 

 They were observed accidentally by the late 

 Thomas Carpenter, Esq., of Tottenham, while 

 making some experiments with a plano-convex 

 jewel lens, employed as the objective of an engis- 

 cope, having a Huyghenian eye-piece. They were 

 then submitted to various instruments, and from the 

 difficulty with which they exhibited the lines even 

 on the larger dark specimens, this object became of 

 great consequence to the microscopist, and some of 

 them were immediately transported, that our neigh- 

 bours the French might try their hands upon 

 them." 



" I have never been able to see the lines on them 

 with a power much below 350. It is also proper to 

 observe that single magnifiers will resolve them, but 

 not without considerable attention is paid to their 

 illumination; but they are most easily made out by 

 the simple light of acandle in the aplanatic engiscope, 

 if .it possesses an angle of 50 degrees, exhibiting all 

 their delicate minutiae with precision. The size of 

 these scales varies from -^ to ,-go of an inch in 

 length, and they decrease in length as they become 

 more transparent. Under a microscope not having 

 sufficient penetration, the tissue appears devoid of 

 markiugs ; but, when placed in a superior one, and 

 the illumination properly made, they show a series 

 of lines or cords on their surface, and present a 

 much greater variety in their arrangement than the 

 scales of any other species of insect. Some have 

 the lines straight (the figure represents the scale 

 with fine longitudinal continuous lines), and have 

 two sets of oblique lines upon them ; others are 

 waved or curved (these lines in the figure are dis- 

 tant, transverse, and irregularly undulate). I must 

 not omit to notice that the cords on these scales are 

 easily rubbed off in mounting." 



We need scarcely observe that these descriptions 

 are erroneous. We now know that there are neither 

 oblique nor transverse lines ; that the longitudina 1 



