ISOl.] MICllOSCOL'lCAL JOIJIINAL. 115 



fluitl as seen by microscopes of very great cost and power fall to the 

 ground unless these peculiar methods are adopted which were then in- 

 vented. Drs. Drysdale and Dallinger not only limited the amount of 

 light, but they fixed a lamp upon a bar on which is placed a mechanical 

 solid rest, by means of which the lamp is brought to one particular posi- 

 tion within the looth of an inch, and out of many positions in which the 

 lamp is placed to get a particular ray of light they found one ray in 

 one particular direction which gave the most superb definition, and in 

 this way they were not only able to see these monads but to distinguish 

 their forms and movements. 



In the year 1884, the optical factory of Carl Zeiss, in Germany, first 

 constructed, under the direction of the world-renowned Professor Abbe, 

 of Germany, microscopic objectives of difi'erent kinds in which perfect 

 correction of the spherical and chromatic aberration was efiected by the 

 use of lenses (one to three in each system) oi /fuorite instead of crtJZf// 

 f^lass. The first experiments in this direction were made by Dr. vSchott, 

 in i88i,and showed clearly the extraordinary technical difficulties in 

 the way of the production of sufiiciently homogeneous glass of such a 

 composition. Fluorite is simpl}^ the continental name for common 

 rtuor-spar. The most common color the mineral assumes is pale green, 

 evident enough in large crystals, but in laminie, thin as the lenses of an 

 objective, scarcely, if at all, perceptible. Of the minerals which occur 

 in nature, cjuartz and calc-spar are the only two which have been reg- 

 ularly used in practical optics. Their chief use depends on the specific 

 optical properties not contained in common glass which they possess. 

 Qiiite dirt'erent points of view are now, however, kept under considera- 

 tion in judging of optical resources for the continued improvement of 

 the compound microscope. The estimation of the materials used in 

 lens-combination has altered in direction ; it has turned to the consider- 

 ation of the degree in which the properties of these materials facilitate 

 and advance the neutralization of the unavoidable subsidiary effects — 

 spherical and chromatic aberration. Having regard to all the condi- 

 tions which regulate the construction of a perfect lens-combination, the 

 spherical aberration in systems of large aperture, as well as the chro- 

 matic aberrations of first and second orders, JJjior-spar affords more 

 profitable relations than any material at present available in optics. 

 The data on which the present conclusions are based were made long 

 ago by the spectroscopic measurements on fluor-spar which Stefan pub- 

 lished in 1871, and those made by Dr. Riedel, of Jena, in the year 1880, 

 and later, at Professor Abbe's instigation, with the use of hydrogen lines 

 on different varieties of the mineral. 



It is little more than a year since the microscopical world was thrillctl 

 by the announcement of an objective made at Jena, the numerical aper- 

 ture of which is 1.63, the highest numerical aperture previously known 

 being t.52. Professor Abbe was four years in bringing this work to 

 completion. Tlie German government had offcned to place a sum equiv- 

 alent to $25,000 at the disposal of any one who would undertake to 

 improve the then existing lenses, and Prof. Abbe, with indefatigable 

 patience, has realized practically all that theory indicated as to the pos- 

 sibilities of the microscope since the discoveries of Goring. 



The mici-oscopical characteristics of the food fats have become a sub- 

 ject of much interest of late vears. My attention was specially directed 



