1 871.] Light. 419 



Cut Lines in Glass." Alluding to his paper on the "Cracks in Silica Films,"* 

 he states that the chances of obtaining perfect illusions are increased by the 

 amount of magnification and the perfection of the objectives employed. The 

 false appearances look more real with well corrected objectives and careful 

 illumination than with bad. Before commencing observations on ruled lines he 

 recommends a careful examination of the edges of a number of thin cover 

 squares, held together and viewed by transmitted light ; the observation is to 

 be conducted with a series of objectives commencing with an inch and proceed- 

 ing upwards to a Jth or £th. Parts of the glasses' edges can easily be focussed to 

 show the true form, but some portions a little in or out of focus will show beads, 

 appearances like columns of Egyptian architecture, &c. Most of these optical 

 appearances are sufficiently hazy or confused to give warning of their true 

 nature, but some may be found so sharp and clear that they might easily mis- 

 lead a practised observer. The lines used were ruled at varying distances : 

 1 — 2000", 1 — 3000", and 1 — 4000". The line cut on glass is defined by Mr. Slack 

 as a furrow, more or less rough at the bottom and sides, and when viewed cor- 

 rectly under the microscope, has the appearance of a narrow depression less 

 transparent than the adjacent spaces. It is difficult to obtain a correct view. 

 The edges of a cut are often apt to appear as two raised lines. The ruled lines, 

 examined in various ways, presented all kinds of appearances ; for instance, 

 the cuts appeared as rounded bands, half-round hollows, with rod-like ridges in 

 the middle ; flattish spaces, with narrow, raised edges. The true appearance 

 was seen only in a single instance. Much is evidently to be done by careful 

 microscopical examination, under varying circumstances of illumination, &c, 

 of objects the true nature of which is known. The value of such researches is 

 clearly shown in Mr. Slack's paper. 



The Royal Microscopical Society has recently acquired an extensive collection 

 of vegetable fibres, chiefly the production of India. The slides are mounted 

 in duplicate, one dry, showing the fibre in its natural condition, the other in 

 balsam, with the ultimate fibres separated, for polariscope observation, by 

 which the structure is better displayed than by any other known means. The 

 collection is similar to one prepared for the India Museum, and is likely to 

 prove of value to those interested in textile materials. Many of the little 

 known Indian fibres are strong, fine, and procurable in large quantities : there 

 exists only the usual prejudice against new materials ; many of them have for 

 years been employed in India for fabrics and cordage, paper-making, and many 

 other purposes. 



The second part of Vol. 160 of the " Philosophical Transactions " contains the 

 long expected account by Dr. G.W. Royston Pigott, M.A., &c.,on "A Searcher 

 for Aplanatic Images applied to Microscopes, and its effects in increasing 

 power and improving definition." The author commences by giving a detailed 

 account of various experiments on the definition of microscopic objectives, 

 which he was led to undertake by an accidental appearance of black beads 

 instead of the usual " I markings" on the Podura scale. At an early period of 

 his researches he abandoned the usual test-objects on account of their real 

 structure being unknown, and formed a test by diminishing some known object, 

 such as a thermometer scale, by means of a high power objective placed in the 

 position of the condenser beneath the stage of the microscope. This aerial 

 or aqueous image, as dry or immersion objectives were used, formed a test 

 readily showing the quality of the definition. Some of the results are 

 surprising, and such as would hardly be expected from what the author calls a 

 " very fine eighth." When the draw-tube was used to increase magnifying 

 power by lengthening the body, it was found that the increase of aberration 

 was in much greater proportion than the increase of magnification. Experiments 

 conducted with the view of ascertaining the nature of these aberrations 

 determined that false images, "eidola" existed above and below the best focus, 

 and that under certain conditions these eidola might become, so to speak, 

 mixed with the true image, giving rise to a great amount of confusion and 



* Monthly Microscopical Journal, vol v„ p. 14. 



