might be used ; but even where those of the shortest focal length are 

 employed, the object whose hnage it is required to form must be 

 placed at a great distance. This would cause various difficulties, and 

 only be practicable with a microscope placed horizontally — unless, 

 indeed, the object selected were very minute, in which case the 

 accurate determination of its diameter (from which that of its image 

 must be afterwards deduced) would be rendered difficult. 



Small air-bells in a fluid are for this purpose far better. I employ 

 by preference a watery solution of powdered gum arable, which 

 always contains numbers of such air-bells originating in the air en- 

 tangled among the particles of the powder. The water employed 

 should have stood for a considerable time freely exposed to the air, 

 or been shaken up with air for some time ; for when we use water 

 which is not saturated with air, the bubbles in the fluid gradually 

 become smaller, and images formed in them decreasing in magnitude, 

 cause errors in the subsequent measurements, as we shall actually 

 find to be the case. 



A drop of the fluid must then be placed on a clean glass object- 

 slide, and covered with a good clear mica plate, a ring-shaped piece 

 of paper being interposed, in order to prevent the flattening of the 

 air-bells by pressure. The object-slide is then placed under the 

 object-glass upon the stage of the microscope, and an air-bell of 

 suitable size for the formation of the images is sought for. All do 

 not give images of the same degree of sharpness ; a peculiarity de- 

 pendent on the fact that some air-bells are in contact with the 

 covering-plate, and consequently have their spherical form disturbed 

 to some extent, or on the presence of small molecules in the fluid 

 above or beneath the air-bell, or even in its interior, causing some 

 haziness of the image, just as defective polish of a glass lens would 

 do. It will, however, be always easy to find some* which will form 

 images of the utmost distinctness and purity. This may be ascertained 

 in the first instance by holding between the mirror and stage some 

 easUy recognized object, e.ff. a piece of paper or the like. The 

 image is always formed on the under surface of the air-bell, which 

 must consequently be brought nearer to the object-glass than when 

 it is desired to bring its margins into focus. 



The object whose image is to be the subject of examination should 

 be placed upon an apparatus, which can be moved upwards and 

 downwards in the space between the mirror and the stage. In some 

 microscopes this can hardly be done, either from the space being too 

 limited, or in consequence of the drum-hke form of the foot of the 

 microscope which quite envelopes the space. If such microscopes, in 



* The following example will demonstrate this. I brought a printed page 

 of a book to such a distance from an air-bell that the length of the image 

 of the whole page was f th milUmetre=about T^oth of an inch, and that of 

 the image of each letter about ^|oth millim.=x2^Troth of an inch. In 

 spite of their minuteness, these images, formed by reflected light, possessed 

 such clearness and sharpness, that under a magnifying power of 154 dia- 

 meters, the whole page was, without difficulty, legible. 



