
THE APERTURE SHUTTER. 89 


whole series of discoveries is said about using it to produce pene- 
tration. This dissertation appears in M. M. J., xi. 17 5, and the 
foot-note clearly sets forth what the instrument was intended for. 
Mr. Slack also mentions this instrument in the M. M. Hee. U0 
234, and those interested in this matter will, no doubt, find this 
paper worthy of perusal, but no mention is made of its producing 
penetration, and the way this term is applied throughout the paper 































Fig. 8. 
leads One to suppose the author really believed penetration to be 
an attribute Jn itself, and had no connection with anything, but 
some mysterious performance known only to a select few. ‘Those 
who remember this lengthy series of articles and the correspondence 
thereon, will know that the doctrine attempted to be laid down, 

