The main objective of this work is to analyse the visual representations (VRs) of the microcosm depicted in nine Greek secondary chemistry school textbooks of the last three decades in order to construct a systemic network for their main conceptual framework and to evaluate the contribution of each one of the resulting categories to the network. The sample comprises a total number of 221 VRs of microcosm, 66 of which are VRs of the 8th grade, 92 of the 9th grade and 63 of the 10th grade. For the qualitative analysis of VRs the phenomenographic method was implemented, whereas a basic quantitative analysis followed. Results provide us with a network that can help science teachers and textbooks designers in identifying the plethora of codes employed in these VRs and the plethora of ways in which VRs can be used, as well as, in determining possible causes of relevant students' misconceptions. Quantitative analysis indicates an effect of grade on the content of VRs and relevant implications for science education are discussed.