330 Royal Society. 



is indebted for this the highest power yet made, produced a sixteenth 

 in the year 1840, and a twenty-sixth in 1860. 



The fiftieth defines even better than the twenty-fifth, which is 

 now made instead of the twenty-sixth. Plenty of light for illu- 

 minating the objects to be examined is obtained by the use of a 

 condenser provided with a thin cap, having an opening not more 

 than the g^oth of an inch in diameter. The preparation may be 

 covered with the thinnest glass made by Messrs. Chance, of Bir- 

 mingham, or with mica, and there is plenty of room for focusing to 

 the lower surface of thin specimens, which can alone be examined by 

 high powers as transparent objects. I beg to draw attention to these 

 very high powers at this time more particularly, because the facts 

 recently urged in favour of the doctrine of spontaneous generation 

 lately revived may be studied with great advantage. Not only are 

 particles, too small to be discerned by a sixteenth, well seen by a 

 twenty-fifth or a fiftieth, but particles too transparent to be observed 

 by the twenty-fifth are distinctly demonstrated by the fiftieth. I 

 feel sure that the further careful study, by the aid of these high 

 powers, of the development and increase of some of the lowest or- 

 ganisms, and the movements which have been seen to occur in con- 

 nexion with certain forms of living matter (Amoeba, white blood- 

 corpuscle, young epithelial cells, &c.), will lead to most valuable 

 results bearing upon the much debated question of vital actions. 



Another very great advantage resulting from the use of the highest 

 powers occurs in minute investigations upon delicate structures which 

 occupy different planes, as is the case in many nervous organs. In 

 studying the distribution of the nerves in some of the peripheral 

 organs of vertebrate animals, very fine fibres can be followed as 

 they lie upon different planes. 



The most delicate constituent nerve-fibres of the plexus in the 

 summit of the papillae of the frog's tongue (*' New Observations upon 

 the Minute Anatomy of the Papillae of the Frog's Tongue," Phil. 

 Trans, for 1864), can be readily traced by the aid of this power. 

 The finest nerve-fibres thus rendered visible are so thin, that in a 

 drawing they would be represented by fine single lines. Near the 

 summit of the papilla there is a very intricate interlacement of nerve- 

 fibres, which, although scarcely brought out by the twenty-fifth, is 

 very clearly demonstrated by this power. In this object the defini- 

 tion of the fibres, as they ramify in various planes one behind another, 

 is remarkable ; and the flat appearance of the specimen as seen by 

 the twenty-fifth, gives place to that of considerable depth of tissue 

 and perspective. The finest nerve-fibres ramifying in the cornea 

 and in certain forms of connective tissue are beautifully brought 

 out by this power, and their relation to the delicate processes from 

 the connective-tissue corpuscles can be more satisfactorily demon- 

 strated than with the twenty-fifth. The advantage of the fiftieth 

 in such investigations seems mainly due to its remarkable power of 

 penetration. The angular aperture of this glass is 150°. Many 

 twelfths have been made with a higher angular aperture, amounting 

 to 170°. 



