812 SUMMAEY OF CURRENT RESEAKCHES RELATING TO 



the passage upwards of the hot-air, and which can be readily closed or 

 opened by the valve without too far swinging. At L is an electro- 

 magnet, which is connected with one pole of the battery and with the 

 regulating thermometer by means of the wire B. The regulating thermo- 

 meter is connected with the other pole of the battery by means of the 

 wire A, 



The action of the apparatus is sufficiently plain. The lamp being 

 lighted, the temperature of the water-bath will rise, and the mercury in the 

 regulating thermometer M with it, until it touches the wire B, thus closing 

 the circuit and magnetizing the electro -magnet, which will attract the 

 lever I, pulling it down, and raising the valve G, so closing the opening in 

 the chimney K, when the heat will escape by the branch chimney F. The 

 temperature of the bath will now fall slightly, and the mercury with it 

 away from B, thus breaking the circuit and demagnetizing the electro- 

 magnet, wliich will cease to attract the lever, and so allow the valve to fall 

 of its own weight, closing the opening into the branch chimney and allowing 

 the Lot air to again ascend through K and reheat the water-bath. This 

 regulating action will continue as long as any oil remains in the lamp, 

 which should therefore have a large reservoir and the flame be turned 

 only high enough to keep the bath slightly above the temperature desired. 

 "With this apparatus many processes such as Weigert's hsematoxylin 

 staining of the nervous system, which, without a constant temperature of 

 long continued duration are impossible of performing, are made easy ; and 

 any one who has had the bother of watching a bath while imbedding in 

 paraffin will appreciate the gain arising from an apparatus which will run 

 all night and have the tissues in good condition for imbedding in the 

 morning, to say nothing of the many other uses, besides staining and im- 

 bedding, to which it can be put." 



Lighton's Analysing Diaphragm for the Polariscope.* — Mr. W. 



Lighten describes this apparatus as follows, stating that he has found it 

 to be of great, help in the study of crystallography. 



" We will suppose that the polariscope as ordinarily used has been placed 

 in position, the polarizing prism below the stage and the analysing prism 

 above the objective. The apparatus consists simply of a cap with movable 

 diaphragm placed over the eye-piece, as illustrated in figs, 223 and 224, 

 Fig, 223 is a sectional view, and fig. 224 a top view of the cap of the eye-piece. 

 The letters in both figs, refer to the same parts. Let AA indicate the axis 

 of the tube ; B, the eye-piece ; C, the cap of ^he eye-piece. The apparatus 

 consists merely in a diaphragm plate D, swinging from right to left on the 

 pivot I. This motion is given by placing the finger at the knob L. The 

 amount of motion is controlled by the two small studs G. The diaphragm 

 is pierced by a small hole H, 1/8 in. in diameter. E is a screw in the top 

 of I, holding the diaphragm in place. F is the apex of the cone of light 

 formed by the image of the source of light passing through the eye-piece. 

 Now, if the diaphragm be so adjusted by sliding the cap upon the eye- 

 piece that it will be on a level with this point of light a very interesting 

 series of optical effects will be observed. The small studs G should be so 

 placed that when the diaphragm is swung to the right or left the sides of 

 the hole H will just cut the axis of the eye- lens (apex of cone of light). 



I will mention a few of the sights seen by its use as described above. 

 In no case were the prisms of the polariscope revolved. A crystal of chlorate 

 of potash was selected, which, upon simply revolving the stage, passed 

 merely from an orange-purple to a dull grey. On introducing the cap and 



* Amer. Mon. Micr. Journ., viii. (1887) pp. 109-10 (2 figs.). 



