INTRODUCTION. xvii 



to relieve the room from unpleasant darkness. They fix 

 impressions upon the brain, so to speak, with a photographic 

 accuracy and fullness of detail not realized in that modern 

 favorite, the blackboard, even with an accomplished 

 draughtsman as lecturer. 



While these projections may not fully take the place of 

 models and physical instruments, they may be largely used 

 in their stead, and, moreover, they can present enlarge- 

 ments of what in them is small, bring into view hidden 

 parts, show in detail what is complex, and so cover the 

 whole ground of an extensive and expensive assortment of 

 other philosophical apparatus, and very much besides 



An Oil-light Sciopticon and case, with a slide-carrier and 

 a glass tank, and two hundred scientific sciopticon slides, 

 showing more than two thousand objects distinct and clear, 

 are all now herein offered for one hundred dollars. These 

 representations are as well classified, and can be as plainly 

 seen, to say the least, as the real objects in the cabinets of 

 any academy of natural science. These sciopticon slides 

 are remarkably clear, and yet vigorous, so as to be suited 

 both for the oil and for the lime-light. Poor slides are 

 sometimes sold at half price, but it is a new thing for the 

 best slides to be offered at less than half price. There must 

 be large demand to justify such reduction. 



The cost of producing each depends much on the quantity 

 made, and the profit depends much on whether the whole 

 production meets with ready sale. As bottom prices and 

 lavish advertisements are incompatible, I venture here to 

 solicit of educators a good word freely spoken by those who 

 know, to fellow-laborers who do not know, the advantages of 

 our apparatus and terms. 



