404 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the same time to extend its magnifying power still more. In this I 

 have sncceeded by employing dilierent objectives for the lower and 

 higher powers, so that it was possible to reduce the height of the 

 apparatus by a third," 



The accompanying figure shows tlio apparatus (Fig. 68), S being 

 a circular column, and T an angular bar, the latter divided into 

 millimetres. G is the drawing plate placed on the box (88 x 

 22 • 5 X 9 • 5 cm.) in which the apparatus packs by separating the 

 pedestal, column and bar, the stage, &c. 



Professor His writes to Dr. Hartnack as follows as to the use of the 

 apparatus : — " Your form is thoroughly serviceable, and allows of 

 correct and convenient working with powers of 4 to 70. According 

 to your request I append some information as to its management. 

 The regulating of the magnifying power is the first thing to be 

 attended to by means of a scale divided into half-millimetres as an 

 object. The stage must be placed in its highest position, and the 

 objective and the })rism moved until the image projected upon the 

 glass plate shows the desired magnifying power. . . . 



" For a power of 4, the stage must be pushed downwards 20 mm., 

 and in order to take in the whole of the field of view with powers 

 of 4 or 5 it must be unscrewed from its ring and the latter used as 

 the stage. 



" The aperture of the stage is only 20 mm. ; short or long- 

 sighted people should always use the same spectacles. When the 

 desired power has been determined the object to be drawn is placed 

 on the stage, and focussed only by moving the latter. In order to 

 obtain a distinct image, the object must be in the same plane as the 

 numbers and strokes of the scale were previously, and if this is 

 obtained by unaltered position of the objective and prism, the 

 magnifying power of the whole apparatus must remain the same as 

 before, the distance of the drawing-surface from the objective remain- 

 ing unchanged." 



[Some general remarks follow as to testing the objectives, the 

 regulation of the light, &c.] 



" Opaque objects are best drawn in liquids. My chief object being 

 to draw embryos, I have had unpolished hollow vessels of black glass 

 or marble made, 5-20 mm. in depth ; the embryos were covered with 

 alcohol and a thin glass plate placed over them in such a manner as 

 to exclude air bubbles. If it is necessary to keep the embryo in a 

 given more or less depressed position, this can be done by using small 

 strips of glass suitably bent. 



" The above directions will perhaps suffice to assist the in- 

 experienced in the use of the apparatus, and I only hope that others 

 may find it, in the elegant and convenient form which you have 

 given it, as useful as I have done." 



Drawing from the Microscope.*^ — Mr. W. T. Suffolk dispenses 

 entirely with the camera lucida, and substitutes a grating ruled in 

 squares and placed over the diaphragm of the eye-piece. It is better 



* Sri. -Gossip, 1882, pp. 49-50. 



