CHAPTER XXIII. 

 USE OF THE OPTICAL LANTERN. 



MANY lecturers and teachers are quite awake to the advan- 

 tages that would arise from the use in lecture and class-rooms 

 of the optical lantern, but almost all are deterred from the use 

 of this valuable instrument by a mistaken idea as to the diffi- 

 culties attending its use. The writer has conversed on this 

 subject with several Professors of note, and in every case the 

 difficulties were either imaginary or exaggerated. 



It is admitted that under certain conditions the student will 

 learn more of the nature of his subject by making a careful, 

 even if faulty, drawing of it as seen in the microscope. But 

 in very many cases the value of drawing is confined to the 

 time actually employed upon the sketch ; that is to say, that 

 whatever is noticed at the time of sketching is learned, but 

 nothing more ; and the sketch thus executed can never teach 

 anything more. A photo-micrograph not only reproduces 

 details independent of the momentary observation of the 

 operator, but it is a litera scripta which is itself open to fur- 

 ther and more leisurely and careful examination, and, more- 

 over, it will probably contain details beyond the power of the 

 hand to copy, even if the eye noticed them ; and if there is one 

 method more apt than another to lead to fresh discoveries, it is 

 the method of enlarging in the optical lantern. 



So much for the student. The advantages of the lantern 

 will be even more sensible to the teacher than the taught. 

 The professor presumably knows what points he wishes to 

 demonstrate, though by a sketch he may be unable to dem- 

 onstrate them ; and every one knows the uncertainty and in- 

 convenience of demonstration to a class by one or more micro- 

 scopes and oral explanations. But let a suitable photograph 

 be produced, a lantern-slide made therefrom, and the image 



