Gibbons^ on a Nciv Method of Micrometry. 35 
The practice of measuring magnifying powers by tlie double 
sight of a scale is not, I believe, peculiar to me, although 
original, and, I think, improved on by my mode ; but the 
essential matter, viz., the construction of scales corre- 
sponding to the magnifying power of the combinations used, 
and the method of viewing such scales with the naked eye 
while the object is viewed through the instrument with the 
other, is absolutely my own ; and I have taken some pains 
to assure myself that it was, as far as I could ascertain, 
new, before I ventured to write a paper on the subject; 
for of course I labour under great disadvantages in this out- 
of-the-way corner of the world, and must be careful that I 
am not hanging on behind while I fancy I am moving ahead. 
I am only surprised that the many persons who have 
laboured toward the simplification of micrometry have not 
hit upon this plan before, as several of them have passed very 
near it. 
My claims, to borrow for a moment the language of the 
Patent Office, are — 
1. The application of permanent magnified scales to the 
magnified image of the object by the use of two eyes, one 
through the instrument and the other outside ; and 
2. The construction of such magnified scales, corresponding 
to given powers, for general use. 
I have only to add that I hope the plans will be found as 
useful and satisfactory to others as they have been to me. I 
shall be glad to find that any makers adopt the suggestions 
I have thrown out, and shall be gratified by hearing from any 
who may undertake the production of such standard scales as 
I have described. 
[That the descriptions in the above paper of the dififerent 
methods of micrometry which have been practised are not in 
every instance correct, the following account of Mr. Lister^s 
method will plainly show : 
He first sketched an outline of the object by means of the 
camera lucida ; and then, without altering the arrangement 
of the microscope, substituted a stage micrometer, and 
sketched a few of its divisions. By subdividing these with 
compasses he formed a scale by which (both being equally 
magnified) the sketch of the object could be measured. 
The author is also in error when he states that all the 
