MEMORANDA. 205 
Some of the most interesting were taken in pairs, so as to be 
shown in the stereoscope, the negatives of which were 
obtained with a -,th of 172° of aperture.* As it is impos- 
sible to see these objects stereoscopically with the full aperture, 
by any form of binocular microscope, it may be stated that 
the pair of negatives were impressed by two different ex- 
posures, slightly shifting the objects sideways and altering 
the focus for each time. In fact, it is difficult to obtain two 
negatives, taken at different times, closely alike. With a 
large-aperture objective, even a difference of illumination 
will give a stereoscopic effect in the combined pictures. 
Some of the diatoms photographed by Dr. Maddox, when 
viewed in the stereoscope, appeared disk-shaped, or with the 
surface of the valve curiously contorted, and the markings 
standing in bold relief, like the seeds of a pomegranate. 
Those who take an interest in the structure of the mark- 
ings on the Diatomaceze may, perhaps, find that a careful 
scrutiny of these photographs (which we believe are now in 
the hands of Mr. How, Foster Lane, Cheapside) will materi- 
ally assist in determining their character, or whether they are 
in the form of projections or depressions.—F. H. Wenuam. 
Notes on Raphides, by George Gulliver, Esq.— We have already 
seen (‘ Annals’ for April and July, 1863), how well this order 
is characterised by raphides, so that not only can a plant 
belonging to it be henceforth truly distinguished from others 
of nearly allied orders by these acicular crystals alone, 
but a minute fragment of the leaf or its modifications may 
be sufficient for the diagnosis; nay, that even a seed-leaf 
would be so was proved in Gnothera and Epilobium. 
This last fact appeared so remarkable that I have lately 
made it the subject of experiments with other plants, when a 
careful examination of numerous species showed that those 
belonging to orders previously ascertained to be regularly 
destitute, in the adult leaves, of raphides, are also equally 
devoid of them in the seed-leaves. Then the seeds of such 
Onagracee as were easily procurable (to wit, Circea lute- 
liana, Eucharidium grandiflorum, Clarkia elegans, C. pul- 
chella, and Godetia vinosa) were sown in pots; and as soon 
as the seed-leaves were well developed above the soil, they 
were all examined, and found in every instance to contain 
raphides. These could be seen both scattered in bundles 
throughout the parenchyma and floating freely and singly 
* The th used was constructed by Mr. Wenham, and lent for the occasion. 
