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Around the beginning of the 1600's, through work attributed to the Janssen brothers (see the microscope in Figure 4) in the Netherlands and Galileo in Italy, the compound microscope was developed. In its simplest form, it consisted of two convex lenses aligned in series: an object glass (objective) closer to the object or specimen; and an eyepiece (ocular) closer to the observer's eye (with means of adjusting the position of the specimen and the microscope lenses). The compound microscope achieves a two-stage magnification. The objective projects a magnified image into the body tube of the microscope and the eyepiece further magnifies the image projected by the objective.
Compound microscopes developed during the 17th and 18th centuries were hampered by optical aberration (both chromatic and spherical), a flaw that is worsened by the use of multiple lenses. These microscopes were actually inferior to single lens microscopes of the period because of these artifacts. The images they produced were often blurred and had the colorful halos associated with chromatic aberrations that not only degrade image quality, but also hamper resolution. In the mid 1700's lens makers discovered that by combining two lenses made of glass with different color dispersions, much of the chromatic aberration could be reduced or eliminated. This discovery was first utilized in telescopes, which have much larger lenses than microscopes. It wasn't until the start of the 1800's that chromatically corrected lenses became commonplace in compound microscopes.
The eighteenth and nineteenth centuries witnessed a great improvement in the mechanical and optical quality of compound microscopes. Advances in machine tools allowed more sophisticated parts to be fabricated and, by the mid 1800's, brass was the alloy of choice for the production of high-quality microscopes. A number of British and German microscope manufacturers flourished during this time period. Their microscopes varied widely in design and production quality, but the overall principles defining their optical properties remained relatively constant. The microscope illustrated in Figure 5 was manufactured by Hugh Powell and Peter Lealand about 1850. The tripod base provided a sturdy support for the microscope, which many people consider the most advanced of its period.
By the end of the 19th century, there was a high degree of competition among microscope manufacturers and the development and production costs of microscopes became an important factor. Brass, the material of choice for microscope manufacturers, is very expensive and it was a lengthy task to machine, polish, and lacquer microscope bodies and other parts machined from this metal. To cut expenses, microscope manufacturers first started to paint the exterior portion of the microscope body and stand, as well as the stage and other non-moving parts.
During the first quarter of the 20th century, many microscope manufacturers had begun substituting cast iron for brass in microscope frames and stages. Iron was much cheaper and could be not be distinguished from brass when painted black. They also started to electroplate many of the critical brass components such as knobs, objective barrels, nosepieces, eyepieces, and mechanical stage assemblies (illustrated in Figure 6). These early 20th century microscopes still subscribed to a common design motif. They were monocular with a substage mirror that was used with an external lamp to illuminate the specimen. A typical microscope of the period is the Zeiss Laboratory microscope pictured in Figure 6. This type of microscope is very functional and many are still in use today.