Page:Hardwicke's Science-Gossip - Volume 1.pdf/146

diffused light of day; use shallow vessels; keep the temperature as even as possible, and between 50° and 58°, and the denisty at 1026.

A some illustration of the success which has followed 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 3½ years in unchanged 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, which have this spring split up into swarms of minute jelly-fishes, or medusæ. We have not spoken of the choice of creatures, and other matters, such as feeding, which would require this paper to be materially lengthened.

Every one accustomed to the use of the microscope knows how much depends on proper illumination in order to ensure successful results. For purposes of microscopic investigation in general, daylight is unquestionably the best source of illumination; it is less trying to the 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, which 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; others like the moderator lamp, and some 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 Belmontine lamp, which combines all the useful properties of the other kinds, and possesses the additional advantage of a flat wick, which is of paramount importance in the exhibition of fine-lined objects. The markings on many of the most difficult forms of the Diatomaceæ 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 Belmontine 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 much light will obliterate the finer structures of delicate and transparent objects; whilst for more opaque objects it is almost impossible to use too brilliant an illumination. For showing the superficial markings on many of the frustules of Diatomaceæ, pencils of rays of great obliquity will be required, especially for the severest test-objects. Without entering upon the question whether these markings be lines or dots, they are evidently produced by inequalities of surface, and are arranged in a linear direction; it follows, consequently, that the best mode of bringing them into view will be to let the light fall upon the object at right angles with its markings, so as to cause the ridges or depressions to cast a shadow in the opposite direction to the side illuminated. The striæ on many forms of Diatomaceæ, that were formerly considered severe tests for objectives of high power, may now be easily resolved with a good ¼ or ⅕ of modern 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 delicate striæ of the more difficult valves, object-glasses of one-eighth or one-twelfth, of large angular aperture, 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 light must be managed so as to have two pencils of rays falling on the object at right angles. This effect may be produced 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 prism, mounted on a separate stand, between the other lamp and the stage of the microscope. This, however, is a cumbersome and bungling performance. The simplest and most successful mode of showing all difficult 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 applicances 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 achromatic 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 varied 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.