194 CLASS DEMONSTRATIONS [Cn. VI 



However, soon after the invention of the compound microscope 

 Kepler and Scheiner discovered the way to get projection pictures r 

 and these have been much used for demonstrating to groups of people 

 the enlarged screen pictures. 



Recently the powerful lime and electric lights have made it possible 

 to carry on these demonstrations to an extent beyond the hopes of 

 the earlier workers; and have put facilities for helping students into 

 the hands of the teacher which are beyond estimation in value. 

 Still for many things and for many persons having charge of large 

 classes the individual simple or compound microscope is still and 

 always will be much used. 



DEMONSTRATION MICROSCOPES AND INDICATORS 

 306. Simple Microscope. Holding the simple microscope in 

 one hand and the specimen in the other has always been used for 

 demonstration, but for class demonstration it is necessary to have 

 microscope and specimen together or the part to be observed by 

 the class is frequently missed. Originally blocks of various kinds to 

 hold both microscope and specimen were devised, but within the 

 last few years excellent pieces of apparatus have been devised by 

 several opticians for the purpose. The accompanying figure shows 

 one of the best forms. 



The tripod magnifier and various pocket magnifiers are excellent 

 for the purpose (fig. 17-18). Where the microscope and object 

 should be held in a fixed position the focusing stand for the simple 

 microscope is good (fig. 19). 



307. Compound demonstration microscope. This was origi- 

 nally called a clinical or pocket microscope. It is thus described by 

 Mayall in his Cantor Lectures on the history of the microscope: 

 "A small microscope was devised by Tolles for clinical purposes 

 which seems to me so good in every way that I must ask special 

 attention for it. The objective is screwed into a sliding tube, and 

 for roughly focusing the sliding motion suffices; for fine adjustment, 

 the sheath is made to turn on a fine screw thread on a cylindrical 

 tube, which serves also as a socket carrier for the stage. The com- 

 pound microscope is here reduced to the simplest form I have met 



