14G THE MICROSCOPE IN THE LECTURE AND CLASS-ROOM. 



room should be of a dark colour. Especially is the proximity of a 

 white ceiling to be avoided. Light is reflected from the screen to 

 such a ceiling, and from it again to the screen, and a great loss of 

 effect is produced. 



The object should be projected on and not passed through 

 the screen. A good, smooth, plastered wall forms the best screen : 

 calico, faced with smooth, white paper, comes next. If the con- 

 ditions are such as to make a translucent screen a necessity, 

 tracing-cloth or tracing-paper will be found to act much more 

 satisfactorily than a wetted sheet. In all cases where a trans- 

 lucent screen is used, all spectators that are near the axial line 

 of the light and lenses see a bright spot of light in the centre of 

 this field of view that greatly detracts from the effect. 



But when we have done our best and obtained the greatest 

 enlargement and definition at present practicable, what does it 

 amount to? Will it enable the lecturer to place before his 

 audience all that he requires to illustrate his subject? Will it 

 place within the reach of the teacher the power to demonstrate to 

 his whole class, at one and the same time, all the special points 

 that call for particular notice ? I am inclined to think that for the 

 lecture-room good photo-micrographs will be found more useful 

 than real objects shown direct on the screen. The lecturer has 

 not often occasion to enter into many of those details of ultimate 

 structure that are so important to the teacher, and a good photo- 

 micrograph, prepared beforehand, and specially arranged to illus- 

 trate the precise point of the lecture, will be more effective when 

 produced before a large audience than a real object when shown 

 on the screen. At the same time, the manipulation of the ordinary 

 lantern is much simpler. 



To the teacher, I think that the oxy-hydrogen microscope, in 

 its best form, offers more advantages. In the first place, it will 

 not be needful to employ so large a screen, and therefore the 

 light will not be so greatly diffused, and the resulting picture will be 

 brighter. Then the ordinary arrangements of a class admit of the 

 more close examination of the image on the screen, and many 

 details that could not have been seen by a large company would 

 be readily seen by a few students, who could closely examine the 

 image on the screen. Still, I have no doubt that there will be 



