260 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



ment packs very conveniently into a small case 5i in. x 2| in. x I4. in. 

 in the manner shown in Fig. 43, and is then readily carried in the pocket. 

 Sir John Lubbock, who has on several botanical excursions taken 

 the instrument with him, speaks highly of its usefulness. 



Excluding Extraneous Light from the Microscope.* — In order to 

 exclude light of an injurious character, whether falling laterally on the 

 eye of the observer or on the stage from above, T. W. Engelmann places 

 the Microscope in a dark bos, made portable, and admitting the light 

 through a funnel-shaped opening in the broad front side. The body 

 of the observer as well as the Microscope and its belongings are 

 intended to be included in the box, which is 75 cm. high, 80 cm. 

 broad, and 40 cm. deep, and is arranged so as to carry accessory 

 apparatus, reagents, coloured glass plates, &c. 



Nachet's Improved Camera Lucida.— In its original form this 

 camera lucida consisted of a rhomboidal prism A B C D, placed 

 over the eye-piece of the Microscope, as shown in Fig. 44, and having 



cemented to the face A C a seg- 

 ment of a small glass cylinder E, 

 the ray ab from the eye-piece 

 and that (a' b') from the pencil 

 meeting the eye at b. 



The disadvantage of this 

 form was that the eye must be 

 held very steadily just over the 

 glass cylinder E (the function 

 of which was to allow the rays 

 from the object to pass to the 

 eye-piece without refraction), to 

 obviate which M. Nachet has 

 made use of a suggestion of 

 Professor G. Govi, and deposits a tliin film of gold on the face 

 A C of the prism (Fig. 45). The gold reflects the ray a' b' to 6 as 



Fig. 44. 



Fig. 45. 



Fig. 46. 



before; whilst, at the same time, on account of its translucency, it 

 allows the ray a to pass through it from the eye-piece. The small 



* Pfliiger's Archiv ges. Physiol., xsiii. (1880) p. 571. 



