1877.] Microscopy. 503 
rotation as much under the actual (mathematical) plane of the object, as 
the glass stratum of the stage would have lifted the ray 
“Convinced, however, by the criticism of competent judges, and by 
my own observations, that the solid glass stage (without central opening) 
offered optical disadvantages which neutralized to a great extent the 
benefits that could be derived from it, I subsequently abandoned glass 
stages of that construction, not, however, before a number of stands had 
been either constructed or were in the course of construction, arranged 
in regard to the hinging point of the illuminating apparatus in such a 
manner as to suit a solid glass stage. The point selected by me for the 
centre of rotation of the illuminating apparatus in these stands would 
have been optically the correct one, if a solid glass stage of my construc- 
tion had been employed. 
“ The stands whose construction was complete at this time, and those 
in process of construction, were not altered, firstly, because it would have 
involved considerable expense to do so, secondly, because I deemed the 
deviation from the actual plane of the object so slight as to be of very 
little consequence, especially as the actual and mathematically correct 
plane of the object is variable, owing to variations in thickness of the 
glass slides, and therefore practically unattainable for the centre of rota- 
tion, unless said centre can be made adjustable to it. 
“Of these stands so made and left unaltered one was sent, with other 
microscope stands of our make, to the Philadelphia exhibition, and was 
there at the opening of the same, and the examination of this stand may 
have given rise to the impression that I intended to place the centre of 
rotation of the illuminating apparatus lower than the plane of the ob- 
ject. The other stands, constructed with a view of using the glass stage 
with central opening, and having the swinging mirror bar hinged slightly 
above the upper surface of the glass stage, were unfortunately not quite 
finished at the time the exhibition opened. 
“ Other stands were then in process of construction, arranged to meet 
the altered circumstances, and were afterwards exhibited at the Centen- 
nial Exhibition in Philadelphia, all of them conceived by me, and exe- 
cuted under my superintendence, before I had seen or heard of Mr. 
Zentmayer’s efforts in the same direction 
SPENCER’S OBJECTIVES. — These aclebented lenses are now made by 
Charles A. Spencer and Sons, at Geneva, N. Y., and sold by G. S. 
Woolman, of New York. In addition to the usual first class series, 
and low angle series, there is an entirely new set called the students’ 
series, of still smaller angle and very low price. 
Excuances. — Shell-sand from the Bermuda Islands, for any really 
valuable material; or selected shells from the same for mountings, of 
special interest. C. C. Merriman, Rochester, N. Y. l 
