16 OPTICAL PRINCIPLES OF THE MICROSCOPE. 



rays from a surface of glass, whether they are entering or are 

 quitting that surface, is much less when they pass from water into 

 glass than from air into glass ; or vice versd, from glass into water 

 than from glass into air. Consequently, when the Object (the 

 frustule of a Diatom for example) is covered with a drop of Water 

 into which the objective dips, there is a much diminished loss of 

 light alike at the surface of the object and at that of the lens ; 

 and in the same manner, when a drop of water is interposed 

 between the front lens of the objective and the covering-glass of 

 an object mounted in balsam or in fluid, there is a much dimin- 

 ished loss of light at each of the glass surfaces. It is of course 

 requisite that the corrections of the Objective should be specially 

 adapted to the course of the rays which enter it from water, 

 instead of from air ; and as the 'immersion-lenses' can therefore 

 only be used as such, they are not universally applicable. It is 

 perhaps in part on this account, and in part because they have 

 succeeded in obtaining equally good results by methods of Illumi- 

 nation which are but little employed on the Continent, that 

 English Opticians have not yet taken up the ' immersion-system.' 

 The Author can bear testimony, however, to the fact that M. 

 Hartnack is able to show most difficult Test Objects by oblique 

 rays simply reflected from the mirror, with a brightness and defini- 

 tion which are only equalled by the best English objectives when 

 used in combination with condensers and stops. And as it is of 

 great advantage in Physiological investigation that the apparatus 

 should be as simple as possible, he would strongly urge the 

 Opticians of London, who are certainly unsurpassed either in 

 theoretical acumen or in manipulative skill, to apply themselves 

 to the production of 'immersion-lenses' that shall at least equal 

 those of Parisian make. 



15. The enlargement of the Angle of Aperture, and the greater 

 completeness of the corrections, first obtained by the adoption of 

 Mr. Lister's principles, soon rendered sensible an imperfection in 

 the performance of these lenses under certain circumstances, which 

 had previously passed unnoticed ; and the important discovery was 

 made by Mr. A. Ross, that a very obvious difference exists in the 

 precision of the image, according as the object is viewed with or 

 without a covering of talc or thin glass ; an Object-glass which is 

 perfectly adapted to either of these conditions, being sensibly 

 defective under the other. The mode in which this difference 

 arises, is explained by Mr. Ross as follows.* Let o, Fig. 11, be 

 any point of an object ; o p the axial ray of the pencil that di- 

 verges from it ; and o T, o t', two diverging rays, the one near to, 

 the other remote from, the axial ray. Now if oaoo represent 

 the section of a piece of thin glass intervening between the object 

 and the object-glass, the rays o t and o t' will be refracted in their 



• " Transactions of the Society of Arts," Vol. li. 



