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Compound achromatic microscope

Year/Period	1870-1890
Maker	CROUCH
Country	United Kingdom
Signature	Henry Crouch / London / 7353
Category	Microscopes
Optical 1	Compound
Optical 2	Achromatic
Size of the box (h x w x d)	15.6 × 30.8 × 18.7 cm
Size of the microscope (h x w x d)	29 × 13.8 × 18 cm
Inventory number	SM 7

Description

In box with lock.
This is a simpler version of the microscope described under SM 5, coarse adjustment is only by means of a sliding tube in a sleeve. Fine adjustment is by a micrometer screw acting on the limb.
There is one Huygenian eyepiece, its focal length is 27.8 mm, its magnification is 9×.
There are two objectives, ‘2/3’ and ‚1/6’, provided with the standard RMS thread. Both were investigated using the Bleeker microscope with a 160 mm body tube and a 5× Huygenian eyepiece.

Objective lens	2/3	1/6
Focal length (mm)	15.57	3.55
Magnification	47.7	243
Numerical aperture	0.15	0.64
Resolving power (µm)	2.5	< 0.8

Objective ‘2/3’ is still in good condition, the results of the star test are good, even its contrast is well defined. Objective ‘1/6’ is not in such a good condition anymore, there is a fault in the cement between two of its lenses. As a result the contrast of the image is not very good and the diffraction rings are not circular. With a Möller diatom test the image is a bit hazy. Stauroneis phoenicenteron is resolved into dots, it is possible that the objective could resolve Pleurosigma angulatum as well when in good condition.