210 Transactions of the Society. 



operates in the present case in consequence of the semi-diameter of 

 the lens being O'S in., representing therefore a condenser 0/10 of 

 an inch in diameter. 



It will be seen that by doubling the thickness of the slide we 

 have lost only one-half per cent, of aperture, and even when trebled 

 the loss is less than 4 per cent. 



As I said above, the aperture of the illuminator is only limited 

 by the refractive index of the flint glass used. If it is desired to 

 have a larger aperture than 1 • (5-44 N.A., it can be obtained by 

 using a flint glass of higher refractive index than 1 • 652. The 

 aperture is alwaj j within half per cent, of the refractive index of the 

 glass (on a slide of the same material " 03 in. thick), so that if 

 glass were used of the refractive index 1*721, the aperture of the 

 illuminator would be 1*712 N.A., and so on to any extent with 

 increasing densities. But when we use glass beyond about 1 * 65 

 we get into difficulties, as very dense glass is always more or less 

 coloured, which, by quenching the more refrangible rays of the 

 spectrum, has a tendency to diminish the effective aperture. It is, 

 moreover, difficult to work, and involves the use of some fluid at 

 least as refractive as itself. With flint of 1 * 65 we have mono- 

 bromide of naphthaline to connect the lens and the slide, and the 

 glass is practically colourless and easy to work ; above all, we have 

 an aperture which exceeds, by 8 per cent., a haham angle of 180". 



To connect tlie ordinary crown glass with the illuminator, the 

 homogeneous medium (oil or otherwise) used with the objective 

 can always be employed, but with flint-glass slides a more refractive 

 fluid, such as oil of cassia or monobromide of naphthaline, is neces- 

 sary. The oil of cassia, if pure, will suffice for an aperture of say 

 1 * 62 N.A., and, in the opinion of most people, it has a less 

 unpleasant smell, but for the full aperture of 1 * 644 this is insuf- 

 ficient, and monobromide or some equally refractive medium must 

 be employed. Such solutions as biniodide of mercury, although 

 inodorous, are objectionable for this purpose, from their chemical 

 action and for other reasons. 



As flint-glass slides are not at present articles of commerce, 

 they must be specially prepared ; but this is not difficult, as disks 

 can be cut of any thickness and polished ; these can be cemented 

 with shellac into circular holes in slides of suitable material, such 

 as vulcanite or ebony. As to the chromatic aberration of the 

 arrangemeut, it is impossible to say anything definite, as in every 

 case, it must depend on the refractive and dispersive powers of the 

 medium in which the object is mounted ; if a condenser is achro- 

 matic on an object in air, it cannot be so in balsam, any more than 

 that which is achromatic in balsam, can be so in phosphorus. 

 Looking merely to chromatic aberration, it may be said that if the 

 imbedding medium has an index exceeding that of the flint glass 



