NOTES AND MEMORANDA. 49 
study of development. He had reached the advanced age of eighty- 
four (84) years. Karl Ernst Von Baér was, though of German 
descent, yet a Russian by birth, having been born toward the close 
of the last century in Esthonia. However, he studied medicine in 
Germany, and in 1817 he received the appointment of Professor of 
Zoology in the University of Kénigsberg, which he held till his return 
to St. Petersburg in 1837. His greatest work, which will make him 
famous for many centuries, is his history of the development of the 
human ovum, and it, which was published at Leipzig in 1827, was of 
course the result of labour executed in the celebrated German 
University. This book is in Latin (4to), and bears the title ‘De Ovi 
Mammalium et Hominis.’ His later essays on development were 
published in the German tongue, and among the more important may 
be mentioned his ‘ Ueber Entwickelungs-geschichte der Fische (4to), 
and his ‘ Ueber die Gefaessverbindung zwischen Mutter und Frucht 
in den Siuge-thieren’ (folio). 
Van der Weyde’s Oblique Illuminator.— Dr. R. H. Ward has 
sent us the following note, contributed by him to the December 
number of the ‘ American Naturalist, and as it bears upon a question 
as to priority of invention, it will doubtless interest our readers :—“ At 
the Indianapolis meeting of the American Association for the Ad- 
vancement of Science, in August, 1871, P. H. Van der Weyde, of New 
York, described a contrivance, believed to be new, for oblique illumi- 
nation of transparent objects. It was designed chiefly to facilitate the 
resolution of lined or dotted objects, and consisted of a plane mirror 
lying beneath the object-slide and parallel to it, from which mirror 
light, condensed upon it from above by means of a bull’s-eye con- 
denser, would be reflected back at the same angle through the object 
and into the objective. These illuminators were shown in successful 
operation at the meeting, working best with moderately high powers, 
and were freely distributed among the members present. They were 
briefly described in the ‘ American Naturalist’ for September, 1871, 
being there estimated as ‘a little expedient of great practical conve- 
nience. yer since that time the present writer, among others, has 
used them habitually, shown them freely, and not unfrequently given 
them away. The mirror may be either of silvered glass or of polished 
metal. In some cases the object-slide may lie directly upon it while 
it rests upon the stage; but frequently the object-slide is best ele- 
vated slightly above it. The mirror is most conveniently made of the 
size of a slide (8x1), and furnished with glass strips at the ends to 
support the slide at any required height ; but it may be made smaller, 
say one inch square or round, and sunken in a brass or wooden stage- 
plate, or for stands having a sub-stage of any kind it may be madeé of 
suitable size and supported from the sub-stage and adjusted for height 
in the same manner as the achromatic condenser. It has the advan- 
tage of great ease of manipulation and applicability to any stand, and 
the drawback of being liable to be interfered with by the presence on 
the slide of such obstructions as paper covers or opaque cells or rings 
E 2 
