242 HOUSE, GARDEN, AND FIELD 



long be provided with the means of demonstrating to a 

 number of pupils simultaneously the most necessary 

 details of animal and plant structure. 



An elementary knowledge of optics or a few trials 

 are necessary to put the objective into its right place. 

 The lantern-objective is removed, and the microscopic 

 objective substituted for it. A stage carrying the object 

 conies outside the objective, and all the parts are placed 

 as in the compound microscope when arranged for work. 

 The next point to be considered is how to absorb a large 

 part of the heat-rays concentrated upon the object, which 

 would soften the mounting medium or scorch the object 

 itself. A glass tank filled with water was first used ; 

 this is liable to the objection that when the water grows 

 warm, bubbles appear, and the water becomes more or 

 less opaque to light. Glycerine was next tried, with far 

 better results. My colleague, Dr. Stroud, suggested that 

 the right liquid to employ is that which is used for mixing 

 with the mounting medium. If, for instance, turpentine 

 or wood spirit is employed to dilute the Canada balsam 

 of the preparation, turpentine or wood spirit must be put 

 into the heat- absorbing tank. The rays which are most 

 readily absorbed by the mounting medium will then be 

 absorbed in advance. We have tried this plan with 

 excellent results, and consider the heat-difficulty as dis- 

 posed of. The tank should be made in one piece, 1 and 

 the operator should remember that turpentine and wood 

 spirit are very inflammable. 2 



The cockroach of the kitchen, which, like the frog or 

 the crayfish, is one of the martyrs of science, may be 

 taken for a first lesson on insect-structure. Distributing 

 dead specimens to the class, we note the external features 

 of an insect. The body is defended by an external armour, 



1 Our tanks were made by the York Glass Co. 



2 It is a pleasure to acknowledge the skill and kindness of my colleague, 

 Dr. Stroud, who devised the simple but excellent lantern-microscope 

 which we now use. 



