300 
NOTES AND MEMORANDA. 
the vernier to the support in which the body slides. I can afiSrm that 
it is of real utility — 
1. When one desires to know the ‘frontal distance’ (that is the 
distance between the anterior surface of the front lens and the point 
where the object ought to be placed) of each of the combinations of 
eye-pieces and objectives which are used. 
2. To measure easily and rapidly the thickness of the covering 
glass — a very important matter in many circumstances. 
3. To find the thickness of the objects examined, &c., and for 
many other cases, too long to be enumerated. 
The vernier which I use is composed of two distinct pieces, 
sliding one against the other ; the one forming the scale is divided 
into millimetres ; the other is the vernier, that is, a piece on which a 
length of nineteen millimetres is divided into twenty equal parts. As 
an example, suppose we want to know the thickness of a covering 
glass. Make a line in ink on one of the faces of the glass, and a line 
on the other side of it. Find under the microscope the visual point of 
one of the sides, and note the reading of the scale. In the same way 
take the visual point of the other line, and the difference between the 
two measures is the thickness.” 
M. Bauwens also describes the way in which he applied a second 
vernier and scale to the draw-tube and body, so as to determine the 
exact distance between the eye-piece and objective. 
Micro-photograph. — Mr. Langenheim, of the United States, has 
photographed the Lord’s Prayer on the ten-thousandth of a square inch, 
and “ so fine that it will bear inspection with a good ^th and B eye- 
piece.” 
