130 



SCIENCE-GOSSIP. 



[Jtoe J, 18G5. 



diffused liglit of day ; use shallow vessels ; keep the 

 temperature as even as possible, and between 50° and 

 58°, and the density at 1026. 



As some illustration of the success which has fol- 

 lowed attention to these hints, we may instance that 

 in a tank and several bowls, we have now, among 

 other creatures, a mullet and a rock blenny, which 

 have lived in good condition for 82 years in un- 

 changed water, and in vessels whose contents have 

 not been disturbed for four years; anemones of 

 various species, for from one to four years ; limpets, 

 three years, and colonies of Hydra tuba, for from 

 three to four years, v/hicli have this spring split up 

 into swarms of minute jelly-fishes, or medusse. IVe 

 have not spoken of the choice of creatures, and 

 other matters, such as feeding, which would require 

 tills paper to be materially lengthened. 



A. PiAMSAY, Jun. 



MICROSCOPIC ILLUMINATION. 



"OYLEY one accustomed to the use of the micro- 

 -^ scope knows how much depends on proper 

 illumination in order to ensure successful results. 

 Por purposes of microscopic investigation in general, 

 daylight is unquestionably the best source of illumi- 

 nation ; it is less trying to tke eyes, and is purer and 

 whiter than artificial light. But as, for obvious 

 reasons, only few persons have the opportunity of 

 using daylight, the question arises, whicli is the best 

 kind of artificial illumination to be employed? Some 

 microscopists give the preference to the Cambridge 

 lamp, which, in its mechanical arrangements, is very 

 convenient; otliers like the moderator lamp, and som.e 

 use the small camphine lamp manufactured expressly 

 for the microscope. Having tried these and almost 

 every other description of artificial illumination, 

 I decidedly give the preference to the common Bel- 

 niontine lamp, which combines aU the useful proper- 

 ties of the other kinds, and possesses the additional 

 advantage of a flat wick, which is of paramount 

 iniportance in the exhibition of fine-lined objects. 

 The markings on many of the most diiiicult forms 

 of the DiatomaccEE may be perfectly displayed by 

 illuminating from the sharp edge of the flame, when 

 by employing its flat surface, or a round flame, not 

 a trace of them would be apparent. It is sometimes 

 urged as an objection to the Belmontiue lamp, that 

 it emits a disagreeable smell, and burns with a 

 smoky flame. These inconveniences may be entirely 

 obviated by a little careful management. All that 

 is required to make it act perfectly is to select a 

 tightly-spun wick, that fits closely into the metal 

 tube in which it runs, being careful to trim the wick 

 before lighting it. 



The character and quantity of light required in 

 microscopic investigation depend very much on the 

 nature of the object to be looked at, for mlich light 



will obliterate the .finer structures of delicate and trans- 

 parent objects; whilst for more opaque objects it is 

 almost impossible to use too brilliant an illumina- 

 tion. Por showing the superficial markings on many 

 of the frustules of Diatomacese, pencils of rays of 

 great obliquity will be required, especially for the 

 severest test-objects. V/ithout entering upon the 

 question whether these markings be lines or dots, 

 they are evidently produced by inequalities of sur- 

 face, and are arranged in a linear direction ; it fol- 

 lows, consequently, that the best mode of bringing 

 them into view will be to let the light fall upon th.e 

 object at right angles with its markings, so as io 

 cause the ridges or depressions to casta shadow in the 

 opposite direction to the side illuminated. The stria3 

 on many forms of Diatomaceaj, that were formerly 

 considered severe tests for objectives of high power, 

 may now be easily resolved with a good i or ^ of mo- 

 dern manufacture, using the concave mirror placed at 

 an angle, so as to reflect rays of considerable obliquity. 

 If, however, it be desired to exhibit the extremely de- 

 licate striaj of the more difficult valves, object-glasses 

 of one-eighth or one-twelfth, of large angular aper- 

 ture, must be employed ; and as most, if not all, of 

 these beautiful and interesting forms have two sets 

 of markings, one running longitudinally and the 

 other crossing in a transverse direction, in order to 

 show both sets simultaneously, the ligh.t must be 

 managed so as to have two pencils of rays falling on 

 the object at right angles. This effect may be pro- 

 duced with two lamps, one placed on the side and 

 the other in front of the microscope, making use of 

 the rays from the mirror for illumination in one 

 direction, and interposing a prisui, mounted on a 

 separate stand, between the other lamp and the 

 stage of the microscope. This, however, is a cum- 

 bersome and bungling performance. The simplest 

 and most successful mode of showing all diflicult 

 tests is by employing an achromatic condenser, 

 such as manufactured by our best opticians, which 

 is fitted with a diaphragm-plate and stops, allowing 

 of every modification in the form and arrangement 

 of the light, and possessing optical appliances so 

 perfect in quality and of such large angular aperture 

 as to afford any amount of obliquity required. 



There is another and much cheaper form of achro- 

 matic condenser, invented by Mr. Webster, of 

 Hanwell, and recently manufactured, which I have 

 seen perform excellently, in resolving difficult test- 

 objects; and, in addition to a large and varird 

 range of capabilities as a condenser, it can be used 

 as a dark ground illuminator with admirable effect. 



But microscopic illumination is just one of those 

 subjects that must be mastered by experiment and 

 patient attention. There are many little niceties 

 which can only be learnt by practice ; one evening's 

 practical attention will teach more than a whole code 

 of written instructions. J. S. 



